feat(docs): First pass at posthog-unity docs (#14090)

* First pass at posthog-unity docs

* Fix available log levels.

Author: haacked

contents/docs/integrate/_snippets/install-unity.mdx

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+PostHog is available for Unity via the [Unity Package Manager](https://docs.unity3d.com/Manual/upm-ui.html).
+
+### Requirements
+
+- Unity 2021.3 LTS or later
+- .NET Standard 2.1 API Compatibility Level
+
+### Via Unity Package Manager (Git URL)
+
+1. Open **Window > Package Manager**
+2. Click the **+** button and select **Add package from git URL**
+3. Enter: `https://github.com/PostHog/posthog-unity.git?path=com.posthog.unity`
+
+### Via local package
+
+1. Clone or download the [posthog-unity repository](https://github.com/PostHog/posthog-unity)
+2. Open **Window > Package Manager**
+3. Click the **+** button and select **Add package from disk**
+4. Navigate to `com.posthog.unity/package.json`
+
+### Configuration
+
+There are two ways to configure PostHog in Unity:
+
+#### Option 1: Unity Inspector (recommended)
+
+The easiest way to configure PostHog is through the Unity Inspector:
+
+1. Go to **Edit > Project Settings > PostHog** (this creates a settings asset if one doesn't exist)
+2. Enter your **API Key** and select your **Host** (US or EU Cloud)
+3. Configure other options as needed
+
+The settings asset is created at `Assets/Resources/PostHogSettings.asset`. PostHog automatically initializes when your game starts using these settings.
+
+You can also create the settings asset manually via **Assets > Create > PostHog > Settings in Resources**.
+
+> **Tip:** Click **Test Connection** in the Inspector to verify your API key is valid.
+
+#### Option 2: Code-based initialization
+
+For more control, initialize PostHog in your game's startup script:
+
+```csharp
+using PostHog;
+using UnityEngine;
+
+public class GameManager : MonoBehaviour
+{
+    void Start()
+    {
+        PostHog.Setup(new PostHogConfig
+        {
+            ApiKey = "<ph_project_api_key>",
+            Host = "<ph_client_api_host>" // usually 'https://us.i.posthog.com' or 'https://eu.i.posthog.com'
+        });
+    }
+}
+```
+
+> **Note:** If you use code-based initialization, remove any `PostHogSettings.asset` from your Resources folder to avoid double initialization.
+
+### Configuration options
+
+All options below can be set via the Unity Inspector or in code:
+
+```csharp
+PostHog.Setup(new PostHogConfig
+{
+    // Required
+    ApiKey = "<ph_project_api_key>",
+
+    // Optional
+    Host = "<ph_client_api_host>",         // PostHog instance URL (default: https://us.i.posthog.com)
+    FlushAt = 20,                           // Events before auto-flush (default: 20)
+    FlushIntervalSeconds = 30,              // Seconds between flushes (default: 30)
+    MaxQueueSize = 1000,                    // Max queued events (default: 1000)
+    MaxBatchSize = 50,                      // Max events per request (default: 50)
+    CaptureApplicationLifecycleEvents = true, // Auto-capture app lifecycle events (default: true)
+    CaptureExceptions = true,               // Auto-capture unhandled exceptions (default: true)
+    PersonProfiles = PersonProfiles.IdentifiedOnly, // When to create person profiles
+    PreloadFeatureFlags = true,             // Fetch flags on init (default: true)
+    SendFeatureFlagEvent = true,            // Track flag usage (default: true)
+    LogLevel = PostHogLogLevel.Warning      // Log verbosity (default: Warning)
+});
+```

contents/docs/integrate/feature-flags-code/_snippets/feature-flags-code-unity.mdx

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+### Boolean feature flags
+
+```csharp
+if (PostHog.IsFeatureEnabled("flag-key"))
+{
+    // Do something differently for this user
+}
+```
+
+### Multivariate feature flags
+
+```csharp
+var flag = PostHog.GetFeatureFlag("flag-key");
+if (flag.IsEnabled)
+{
+    string variant = flag.GetVariant("control"); // "control" is the default value
+    if (variant == "variant-key") // Replace with your variant key
+    {
+        // Do something for this variant
+    }
+}
+```
+
+### Feature flag payloads
+
+Access payloads through the feature flag object:
+
+```csharp
+// Define your payload class (must use [Serializable] and public fields for JsonUtility)
+[Serializable]
+public class CheckoutConfig
+{
+    public string theme;
+    public int maxItems;
+    public bool showBanner;
+}
+
+// Get flag and access payload
+var flag = PostHog.GetFeatureFlag("checkout-config");
+if (flag.IsEnabled)
+{
+    var config = flag.GetPayload<CheckoutConfig>();
+    Debug.Log($"Theme: {config.theme}, Max items: {config.maxItems}");
+}
+
+// For dynamic/nested payloads, use PostHogJson
+var payload = flag.GetPayloadJson();
+string theme = payload["theme"].GetString("light");
+int maxItems = payload["settings"]["maxItems"].GetInt(10);
+```
+
+### Ensuring flags are loaded before usage
+
+Every time a user opens the app, we send a request in the background to fetch the feature flags that apply to that user. We store those flags in storage.
+
+This means that for most screens, the feature flags are available immediately – **except for the first time a user visits**.
+
+To be notified when flags are loaded:
+
+```csharp
+// Via callback during setup
+PostHog.Setup(new PostHogConfig
+{
+    ApiKey = "<ph_project_api_key>",
+    OnFeatureFlagsLoaded = () => Debug.Log("Flags ready!")
+});
+
+// Or via event
+PostHog.OnFeatureFlagsLoaded += () => UpdateUI();
+```
+
+### Reloading feature flags
+
+Feature flag values are cached. If something has changed with your user and you'd like to refetch their flag values, call:
+
+```csharp
+await PostHog.ReloadFeatureFlagsAsync();
+```
+
+### Overriding server properties for flags
+
+You can set properties used for flag evaluation:
+
+```csharp
+// Set person properties for targeting
+PostHog.SetPersonPropertiesForFlags(new Dictionary<string, object>
+{
+    { "plan", "premium" },
+    { "beta_user", true }
+});
+
+// Set group properties for targeting
+PostHog.SetGroupPropertiesForFlags("company", new Dictionary<string, object>
+{
+    { "size", "enterprise" }
+});
+
+// Reset properties
+PostHog.ResetPersonPropertiesForFlags();
+PostHog.ResetGroupPropertiesForFlags();
+```

contents/docs/integrate/send-events/_snippets/send-events-unity.mdx

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+import SettingProperties from "./setting-properties-text.mdx"
+import NamingTip from "./naming-tip.mdx"
+import Intro from "./intro.mdx"
+
+<Intro />
+
+```csharp
+PostHog.Capture("user_signed_up");
+```
+
+<NamingTip />
+
+<SettingProperties />
+
+```csharp
+PostHog.Capture("user_signed_up", new Dictionary<string, object>
+{
+    { "login_type", "email" },
+    { "is_free_trial", true }
+});
+```
+
+### Capturing screen views
+
+Track screen views to understand user navigation through your game:
+
+```csharp
+PostHog.Screen("Main Menu");
+
+// With additional properties
+PostHog.Screen("Level Select", new Dictionary<string, object>
+{
+    { "unlocked_levels", 5 }
+});
+```