[docs] Move markdown style guide to chromium docs

It's currently hosted at [chromium project][1] page, which is difficult
to find compared to other style guides listed here.

Will submit another change to redirect from the existing page.

[1]: https://www.chromium.org/developers/markdown-documentation/

Change-Id: Id00ebb7960e9d41e91d6623df9b2d512ee03677b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4302569
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Auto-Submit: Ming-Ying Chung <mych@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1112579}

docs/README.md

@@ -20,7 +20,8 @@ below.**
     and the
     [Chromium Documentation Best Practices](documentation_best_practices.md).
 *   Markdown documents must follow the
-    [style guide](https://github.com/google/styleguide/tree/gh-pages/docguide).
+    [Markdown Style
+    Guide](https://chromium.googlesource.com/chromium/src/+/HEAD/styleguide/markdown/markdown.md).
 
 ### Previewing changes
 

docs/navbar.md

@@ -10,7 +10,7 @@
 * [Design Docs](/docs/design/README.md)
 * [Contact](https://www.chromium.org/contact/)
 * [Bugs](https://www.chromium.org/for-testers/bug-reporting-guidelines/)
-* [Markdown Style Guide](https://github.com/google/styleguide/tree/gh-pages/docguide)
+* [Style Guide](https://chromium.googlesource.com/chromium/src/+/main/styleguide/styleguide.md)
 * [Markdown Syntax](https://gerrit.googlesource.com/gitiles/+/HEAD/Documentation/markdown.md)
 * [Old Docs](https://www.chromium.org/Home/)
 * [Search](https://source.chromium.org/chromium/chromium/src/+/main:docs/)

styleguide/markdown/markdown.md

@@ -0,0 +1,31 @@
+# Markdown Style Guide in Chromium
+
+## Style Guide
+
+Please write your Markdown file
+
+*   In accordance with [Google Documentation
+    Guide](https://github.com/google/styleguide/tree/gh-pages/docguide)
+*   [Limit lines to **80 characters** where
+    possible](https://groups.google.com/a/chromium.org/d/msg/chromium-dev/KECdEn562vY/sqRor1frEgAJ)
+
+Unfortunately, `git cl format` will NOT do this for you.
+
+## Documentation Guide
+
+New documentation related to code structure should be put in Markdown. The best
+practices recommendation is to put a `README.md` file in the code directory
+closest to your component or code.
+
+See [Chromium Documentation
+Guidelines](https://chromium.googlesource.com/chromium/src/+/HEAD/docs/documentation_guidelines.md)
+for details.
+
+[Here are some
+examples](https://cs.chromium.org/search/?q=file:readme.md+-file:/third_party/)
+to learn from.
+
+## Making Changes
+
+See [Creating
+Documentation](https://chromium.googlesource.com/chromium/src/+/HEAD/docs/README.md#creating-documentation).

styleguide/styleguide.md

@@ -14,7 +14,7 @@
     for build files.
     *   See also: [Writing GN templates](../build/docs/writing_gn_templates.md)
         for Chromium best-practices.
-*   [Markdown guide](https://www.chromium.org/developers/markdown-documentation)
+*   [Markdown style guide](markdown/markdown.md)
 
 Chromium also uses these languages to a lesser degree: