Document Docusaurus trailingSlash requirement for Read the Docs (#12583)

Docusaurus requires `trailingSlash: true` to be explicitly set to work
correctly on Read the Docs. When not set or set to `false`, users must
configure redirects manually.

## Changes

Added "Configure trailing slashes" section to
`/docs/user/intro/docusaurus.rst` explaining:
- `trailingSlash: true` must be explicitly configured for proper URL
resolution (`/docs/intro` → `/docs/intro/index.html`)
- Without this setting (or if set to `false`), manual redirect
configuration is required

```js
// docusaurus.config.js
export default {
    // Required for compatibility with Read the Docs
    trailingSlash: true,
};
```

Follows the same pattern as the Markdoc documentation.

> [!WARNING]
>





<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
>
<__filter_complete__></__filter_complete__></details><issue_title>Docs:
mention that Docusaurus needs `trailingSlash: true` (default) to
work</issue_title>
><issue_description>We are documenting Docusaurus at
https://docs.readthedocs.com/platform/latest/intro/docusaurus.html
> 
> We should mention that `trailingSlash: true` is required to work
without issues. When it's `false`, the user needs to define a redirect:
https://github.com/readthedocs/readthedocs.org/pull/12323</issue_description>
> 
><agent_instructions>Just update the docs in the docs/user
repo</agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
><comments>
></comments>
>

- Fixes #12345

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Docs: mention that Docusaurus needs `trailingSlash: true`
(default) to work</issue_title>
> <issue_description>We are documenting Docusaurus at
https://docs.readthedocs.com/platform/latest/intro/docusaurus.html
> 
> We should mention that `trailingSlash: true` is required to work
without issues. When it's `false`, the user needs to define a redirect:
https://github.com/readthedocs/readthedocs.org/pull/12323</issue_description>
> 
> <agent_instructions>Just update the docs in the docs/user
repo</agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #12345

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Closes #12345

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ericholscher <25510+ericholscher@users.noreply.github.com>

Author: Copilot

docs/user/intro/docusaurus.rst

@@ -62,6 +62,22 @@ For optimal integration with Read the Docs, make the following optional configur
    :local:
    :backlinks: none
 
+Configure trailing slashes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For proper operation on Read the Docs, you need to set ``trailingSlash: true`` in your Docusaurus configuration.
+This ensures that URLs like ``/docs/intro`` are handled as ``/docs/intro/`` and the corresponding ``index.html`` file is served correctly.
+
+Without this setting (or if set to ``false``), you will need to configure redirects to ensure proper URL resolution.
+
+.. code-block:: js
+    :caption: docusaurus.config.js
+
+    export default {
+        // Required for compatibility with Read the Docs
+        trailingSlash: true,
+    };
+
 Set the canonical URL
 ~~~~~~~~~~~~~~~~~~~~~