[D1] Adding information on how wrangler.toml is structured behind the scenes. (#18145)

* Adding information on how wrangler.toml is structured behind the scenes.

* Changing names to be more specific/relevant.

* Fixing formatting. Clarifying wording.

* Wording clarity

* Update src/content/docs/d1/configuration/environments.mdx

Co-authored-by: Lambros Petrou <[email protected]>

* Replacing tabs with spaces.

* Update src/content/docs/d1/configuration/environments.mdx

Co-authored-by: Pedro Sousa <[email protected]>

---------

Co-authored-by: Lambros Petrou <[email protected]>
Co-authored-by: Pedro Sousa <[email protected]>

Author: 3 people

src/content/docs/d1/configuration/environments.mdx

@@ -25,3 +25,77 @@ d1_databases = [
 ```
 
 In the code above, the `staging` environment is using a different database (`DATABASE_NAME_1`) than the `production` environment (`DATABASE_NAME_2`).
+
+## Anatomy of `wrangler.toml` file
+
+If you need to specify different D1 databases for different environments, your `wrangler.toml` may contain bindings that resemble the following:
+
+```toml
+[[production.d1_databases]]
+binding = "DB"
+database_name = "DATABASE_NAME"
+database_id = "DATABASE_ID"
+```
+
+In the above configuration:
+
+- `[[production.d1_databases]]` creates an object `production` with a property `d1_databases`, where `d1_databases` is an array of objects, since you can create multiple D1 bindings in case you have more than one database.
+- Any property below the line in the form `<key> = <value>` is a property of an object within the `d1_databases` array.
+
+Therefore, the above binding is equivalent to:
+
+```json
+{
+  "production": {
+    "d1_databases": [
+      {
+        "binding": "DB",
+        "database_name": "DATABASE_NAME",
+        "database_id": "DATABASE_ID"
+      }
+    ]
+  }
+}
+```
+
+### Example
+
+```toml
+[[env.staging.d1_databases]]
+binding = "BINDING_NAME_1"
+database_name = "DATABASE_NAME_1"
+database_id = "UUID_1"
+
+[[env.production.d1_databases]]
+binding = "BINDING_NAME_2"
+database_name = "DATABASE_NAME_2"
+database_id = "UUID_2"
+
+```
+
+The above is equivalent to the following structure in JSON:
+
+```json
+{
+  "env": {
+    "production": {
+      "d1_databases": [
+        {
+          "binding": "BINDING_NAME_2",
+          "database_id": "UUID_2",
+          "database_name": "DATABASE_NAME_2"
+        }
+      ]
+    },
+    "staging": {
+      "d1_databases": [
+        {
+          "binding": "BINDING_NAME_1",
+          "database_id": "UUID_1",
+          "database_name": "DATABASE_NAME_1"
+        }
+      ]
+    }
+  }
+}
+```