docs: clarify the usage of slug (#6926)

* Clarify the usage of slug

As a new user it was unclear whether setting `slug` would change the URL relative to the root directory or relative to the docs directory. The example I added should make that clear without needing to test out the functionality in a Docusaurus instance (which is what I had to do).

* editorial changes

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

website/docs/guides/docs/docs-introduction.md

@@ -37,17 +37,31 @@ id: part1
 Lorem ipsum
 ```
 
-If you want more control over the last part of the document URL, it is possible to add a `slug` (defaults to the `id`).
+### Customizing doc URLs {#customizing-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.
+
+For example, suppose your site structure looks like this:
+
+```bash
+website # Root directory of your site
+└── docs
+    └── guide
+        └── hello.md
+```
+
+By default `hello.md` will be available at `/docs/guide/hello`. You can change its URL location to `/docs/bonjour`:
 
 ```md
 ---
-id: part1
+slug: /bonjour
-slug: part1.html
 ---
 
 Lorem ipsum
 ```
 
+`slug` will be appended to the doc plugin's `routeBasePath`, which is `/docs` by default. See [Docs-only mode](#docs-only-mode) for how to remove the `/docs` part from the URL.
+
 :::note
 
 It is possible to use: