Add content collection custom IDs section (#10109)

Co-authored-by: sarahrainsberger <sarahrainsberger@gmail.com>
Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>
Co-authored-by: ascorbic <213306+ascorbic@users.noreply.github.com>
Co-authored-by: sarah11918 <5098874+sarah11918@users.noreply.github.com>

src/content/docs/en/guides/content-collections.mdx

@@ -85,9 +85,9 @@ The Content Layer API allows you to fetch your content (whether stored locally i
 
 Astro provides two built-in loader functions (`glob()` and `file()`) for fetching your local content, as well as access to the API to construct your own loader and fetch remote data.
 
-The `glob()` loader creates entries from directories of Markdown, MDX, Markdoc, or JSON files from anywhere on the filesystem. It accepts a `pattern` of entry files to match, and a `base` file path of where your files are located. Use this when you have one file per entry.
+The `glob()` loader creates entries from directories of Markdown, MDX, Markdoc, or JSON files from anywhere on the filesystem. It accepts a `pattern` of entry files to match, and a `base` file path of where your files are located. Each entry's `id` will be automatically generated from its file name. Use this loader when you have one file per entry.
 
-The `file()` loader creates multiple entries from a single local file. It accepts a `base` file path to your file and optionally a [`parser` function](#parser-function) for data files it cannot parse automatically. Use this when your data file can be parsed as an array of objects.
+The `file()` loader creates multiple entries from a single local file. Each entry in the file must have a unique `id` key property. It accepts a `base` file path to your file and optionally a [`parser` function](#parser-function) for data files it cannot parse automatically. Use this loader when your data file can be parsed as an array of objects.
 
 ```ts  title="src/content.config.ts" {5,9}
 import { defineCollection, z } from 'astro:content';
@@ -337,6 +337,28 @@ relatedPosts:
 ---
 ```
 
+### Defining custom IDs
+
+When using the `glob()` loader with Markdown, MDX, Markdoc, or JSON files, every content entry [`id`](/en/reference/modules/astro-content/#id) is automatically generated in an URL-friendly format based on the content filename. The `id` is used to query the entry directly from your collection. It is also useful when creating new pages and URLs from your content.
+
+You can override an entry’s generated `id` by adding your own `slug` property to the file frontmatter or data object for JSON files. This is similar to the “permalink” feature of other web frameworks.
+
+```md title="src/blog/1.md" {3}
+---
+title: My Blog Post
+slug: my-custom-id/supports/slashes
+---
+Your blog post content here.
+```
+
+```json title="src/categories/1.json" {3}
+{
+  "title": "My Category",
+  "slug": "my-custom-id/supports/slashes",
+  "description": "Your category description here."
+}
+```
+
 ## Querying Collections
 
 Astro provides helper functions to query a collection and return one (or more) content entries.