docs: emphasize "index slug" convention (#9906)

website/docs/guides/docs/docs-create-doc.mdx

@@ -112,7 +112,21 @@ The ID is used to refer to a document when hand-writing sidebars, or when using
 
 ### Doc URLs {#doc-urls}
 
-By default, a document's URL location is its file path relative to the `docs` folder. Use the `slug` front matter to change a document's URL.
+By default, a document's URL location is its file path relative to the `docs` folder, with a few exceptions. Namely, if a file is named one the following, the file name won't be included in the URL:
+
+- Named as `index` (case-insensitive): `docs/Guides/index.md`
+- Named as `README` (case-insensitive): `docs/Guides/README.mdx`
+- Same name as parent folder: `docs/Guides/Guides.md`
+
+In all cases, the default slug would only be `/Guides`, without the `/index`, `/README`, or duplicate `/Guides` segment.
+
+:::note
+
+This convention is exactly the same as [the category index convention](./sidebar/autogenerated.mdx#category-index-convention). However, the `isCategoryIndex` configuration does _not_ affect the document URL.
+
+:::
+
+Use the `slug` front matter to change a document's URL.
 
 For example, suppose your site structure looks like this: