Add mdbook docs page (#11849)

This shows how to host docs with mdbook.
    
Requires #11848 to
not need a reshim (example:
https://github.com/readthedocs/test-builds/blob/mdbook/.readthedocs.yaml#L10)


<!-- readthedocs-preview docs start -->
---
:books: Documentation previews :books:

- User's documentation (`docs`):
https://docs--11849.org.readthedocs.build/en/11849/

<!-- readthedocs-preview docs end -->

<!-- readthedocs-preview dev start -->
- Developer's documentation (`dev`):
https://dev--11849.org.readthedocs.build/en/11849/

<!-- readthedocs-preview dev end -->

---------

Co-authored-by: Manuel Kaufmann <[email protected]>

Author: ericholscher

docs/user/intro/doctools.rst

@@ -55,10 +55,21 @@ Below is a list of popular documentation tools that you can use to write your do
         Written in
              :bdg-info:`javascript`
 
+    .. grid-item-card::  mdBook
+        :link: mdbook.html
+
+        mdBook is a command line tool to create books with Markdown built in Rust.
+
+        Supported formats
+             :bdg-success:`md`
+        Written in
+             :bdg-info:`rust`
+
 .. toctree::
    :hidden:
 
    /intro/mkdocs
    /intro/sphinx
    /intro/docusaurus
    /intro/markdoc
+   /intro/mdbook

docs/user/intro/mdbook.rst

@@ -0,0 +1,71 @@
+mdBook
+======
+
+.. meta::
+   :description lang=en: Learn how to host mdBook documentation on Read the Docs.
+
+`mdBook`_ is a command line tool to create books with Markdown.
+
+Minimal configuration is required to build an existing mdBook project on Read the Docs.
+
+.. code-block:: yaml
+   :caption: .readthedocs.yaml
+
+    version: 2
+
+    build:
+        os: ubuntu-lts-latest
+        tools:
+            rust: latest
+        commands:
+            - cargo install mdbook
+            # For an example book..
+            # - mdbook init docs
+            - mdbook build docs --dest-dir $READTHEDOCS_OUTPUT/html
+
+.. _mdBook: https://rust-lang.github.io/mdBook/
+
+Getting started
+---------------
+
+- If you have an existing mdBook project you want to host on Read the Docs, check out our :doc:`/intro/add-project` guide.
+- If you're new to mdBook, check out the official `Getting started with mdBook`_ guide.
+
+.. _Getting started with mdBook: https://rust-lang.github.io/mdBook/guide/creating.html
+
+Configuring mdBook and Read the Docs Addons
+-------------------------------------------
+
+Adjust the flyout menu font size
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Add a ``readthedocs.css`` to your build with the `additional-css <https://crisal.io/tmp/book-example/book/format/config.html#html-renderer-options>`_ flag,
+so that the font in the :ref:`flyout-menu:Addons flyout menu` matches the theme better.
+
+.. code-block:: css
+    :caption: readthedocs.css:
+
+    :root {
+        /* Increase the font size of the flyout menu */
+        --readthedocs-flyout-font-size: 1.3rem;
+
+        /* Increase the font size of the notifications */
+        --readthedocs-notification-font-size: 1.3rem;
+
+    }
+
+Example repository and demo
+---------------------------
+
+Example repository
+    https://github.com/readthedocs/test-builds/tree/mdbook
+
+Demo
+    https://test-builds.readthedocs.io/en/mdbook/
+
+Further reading
+---------------
+
+* `mdBook documentation`_
+
+.. _mdBook documentation: https://rust-lang.github.io/mdBook/

docs/user/intro/mkdocs.rst

@@ -161,7 +161,8 @@ To integrate the :ref:`flyout-menu:Addons flyout menu` version menu into your si
 Adjust the flyout menu font size
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Edit ``readthedocs.css`` to so that the font in the :ref:`flyout-menu:Addons flyout menu` matches the theme better.
+Add a ``readthedocs.css`` to your build,
+so that the font in the :ref:`flyout-menu:Addons flyout menu` matches the theme better.
 
 .. code-block:: css
     :caption: readthedocs.css: