[AIG]headers glossary initial documentation (#18250)

* headers glossary initial documentation

* minor changes

* Update glossary.mdx

* added links

* added links

* added header hierachy docs

* spelling

* added links

* Moved hierarchy info

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Co-authored-by: Kathy <[email protected]>

Author: daisyfaithauma

src/content/docs/ai-gateway/glossary.mdx

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+---
+title: Glossary
+pcx_content_type: glossary
+---
+
+import { Glossary } from "~/components";
+
+AI Gateway supports a variety of headers to help you configure, customize, and manage your API requests. This page provides a complete list of all supported headers, along with a short description
+
+<Glossary product="ai-gateway" />
+
+## Configuration hierarchy
+
+Settings in AI Gateway can be configured at three levels: **Provider**, **Request**, and **Gateway**. Since the same settings can be configured in multiple locations, the following hierarchy determines which value is applied:
+
+1. **Provider-level headers**:
+   Relevant only when using the [Universal Endpoint](/ai-gateway/providers/universal/), these headers take precedence over all other configurations.
+2. **Request-level headers**:
+   Apply if no provider-level headers are set.
+3. **Gateway-level settings**:
+   Act as the default if no headers are set at the provider or request levels.
+
+This hierarchy ensures consistent behavior, prioritizing the most specific configurations. Use provider-level and request-level headers for more fine-tuned control, and gateway settings for general defaults.

src/content/glossary/ai-gateway.yaml

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+---
+productName: AI Gateway
+entries:
+  - term: cf-aig-event-id
+    general_definition: |-
+      [cf-aig-event-id](/ai-gateway/evaluations/add-human-feedback-api/#3-retrieve-the-cf-aig-log-id) is a unique identifier for an event, used to trace specific events through the system.
+
+  - term: cf-aig-step
+    general_definition: |-
+      [cf-aig-step](/ai-gateway/configuration/fallbacks/#response-headercf-aig-step) identifies the processing step in the AI Gateway flow for better tracking and debugging.
+
+  - term: cf-aig-log-id
+    general_definition: |-
+      The [cf-aig-log-id](/ai-gateway/evaluations/add-human-feedback-api/#3-retrieve-the-cf-aig-log-id) is a unique identifier for the specific log entry to which you want to add feedback.
+
+  - term: cf-aig-collect-log
+    general_definition: |-
+      The [cf-aig-collect-log](/ai-gateway/observability/logging/#collect-logs-cf-aig-collect-log) header allows you to bypass the default log setting for the gateway.
+
+  - term: cf-aig-custom-cost
+    general_definition: |-
+      Allows the [customization of request cost](/ai-gateway/configuration/custom-costs/#custom-cost) to reflect user-defined parameters.
+
+  - term: cf-aig-cache-key
+    general_definition: |-
+      The [cf-aig-cache-key-aig-cache-key](/ai-gateway/configuration/caching/#custom-cache-key-cf-aig-cache-key) let you override the default cache key in order to precisely set the cacheability setting for any resource.
+
+  - term: cf-aig-cache-status
+    general_definition: |-
+      [Status indicator for caching](/ai-gateway/configuration/caching/#default-configuration), showing if a request was served from cache.
+
+  - term: cf-aig-metadata
+    general_definition: |-
+      [Custom metadata](/ai-gateway/configuration/custom-metadata/)allows you to tag requests with user IDs or other identifiers, enabling better tracking and analysis of your requests.
+
+  - term: cf-aig-cache-ttl
+    general_definition: |-
+      Specifies the [cache time-to-live for responses](/ai-gateway/configuration/caching/#cache-ttl-cf-aig-cache-ttl).
+
+  - term: cf-aig-skip-cache
+    general_definition: |-
+      Header to [bypass caching for a specific request](/ai-gateway/configuration/caching/#skip-cache-cf-aig-skip-cache).
+
+  # Deprecated headers
+  - term: cf-cache-ttl
+    general_definition: |-
+      Deprecated: This header is replaced by `cf-aig-cache-ttl`. It specifies cache time-to-live.
+
+  - term: cf-skip-cache
+    general_definition: |-
+      Deprecated: This header is replaced by `cf-aig-skip-cache`. It bypasses caching for a specific request.