Docs: add API pagination guide

docs/api/api-pagination.md

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+# API Pagination
+##
+
+All Private Packagist API endpoints that return lists support pagination using query parameters. This allows you to efficiently retrieve large result sets by fetching data in smaller chunks.
+
+> **✨ Automatic Pagination**: If you're using the [PHP API client](https://github.com/packagist/private-packagist-api-client), pagination is handled automatically! The client fetches all pages transparently and returns complete results. You don't need to manually handle pagination, Link headers, or page loops - just call the endpoint method and get all results.
+
+If you were previously using list endpoints without pagination: Your existing code will continue to work, but the hard limit is set to 10,000 items, which covers most use cases.
+
+## Query Parameters
+
+Paginated list endpoints (all that do get a collection of items) accept the following query parameters:
+
+* `page` (integer, optional): Page number to retrieve (1-10,000). Default: 1
+* `limit` (integer, optional): Number of items per page (1-10,000). Default: 10,000
+
+### Example Request
+
+```bash
+GET /api/packages/?page=2&limit=300
+```
+
+This fetches the second page of packages with 300 packages per page.
+
+## Response Format
+
+### Link Header
+
+Paginated responses include an [RFC 5988](https://tools.ietf.org/html/rfc5988) Link header containing pagination navigation URLs:
+
+```
+Link: <https://packagist.com/api/packages/?page=3&limit=300>; rel="next",
+      <https://packagist.com/api/packages/?page=1&limit=300>; rel="prev",
+      <https://packagist.com/api/packages/?page=1&limit=300>; rel="first",
+      <https://packagist.com/api/packages/?page=10&limit=300>; rel="last"
+```
+
+The Link header includes up to four relations:
+
+* `next`: URL to the next page (if available)
+* `prev`: URL to the previous page (if available)
+* `first`: URL to the first page
+* `last`: URL to the last page
+
+### Response Body
+
+The response body contains a JSON array of resources, just like non-paginated responses:
+
+```json
+[
+  {
+    "id": 1,
+    "name": "acme/package",
+    ...
+  },
+  ...
+]
+```
+
+## Using the PHP Client
+
+The PHP client includes an AutoPaginator plugin that:
+
+1. **Automatically detects pagination**: Checks for Link headers with `rel="next"` in API responses
+2. **Fetches all pages**: Makes additional requests to retrieve all pages (using 500 items per page)
+3. **Merges results**: Combines all pages into a single array
+4. **Returns complete data**: Your code receives all results, no matter how many pages exist