Add mdbook docs page

This shows how to host docs with mdbook.

Requires #11848 to not need a reshim

Author: ericholscher

docs/user/intro/doctools.rst

@@ -49,3 +49,13 @@ with more coming soon.
              :bdg-success:`md`
         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`

docs/user/intro/mdbook.rst

@@ -0,0 +1,57 @@
+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 --version
+            - 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/
+
+Limitations
+-----------
+
+All Read the Docs features are supported for mdBook projects,
+as it builds static HTML files that can be hosted on Read the Docs.
+
+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
+
+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/