docs: restore docs for Bash/Zsh completion

The docs were removed seemingly by accident.

Also change Markdown headers with <a> tags to HTML
<hN> tags with IDs, because the Jekyll version we
use to render .md to .html ignores the <a> tags
and respects only the <hN> tags when creating
links to headings.

See https://github.com/bazelbuild/bazel/issues/4255#issuecomment-432120127

Fixes https://github.com/bazelbuild/bazel/issues/4255

Closes #6472.

Change-Id: I024e1d5025849db3a1484a41d58287b59973a33f
PiperOrigin-RevId: 218679796
diff --git a/site/_layouts/documentation.html b/site/_layouts/documentation.html
index 04b2fd4..77410c6 100644
--- a/site/_layouts/documentation.html
+++ b/site/_layouts/documentation.html
@@ -47,6 +47,7 @@
                    <li><a href="/versions/{{ site.version }}/install-os-x.html">Installing on macOS</a></li>
                    <li><a href="/versions/{{ site.version }}/install-windows.html">Installing on Windows</a></li>
                    <li><a href="/versions/{{ site.version }}/install-compile-source.html">Compiling from Source</a></li>
+                   <li><a href="/versions/{{ site.version }}/completion.html">Command-Line Completion</a></li>
                    <li><a href="/versions/{{ site.version }}/ide.html">Integrating with IDEs</a></li>
                  </ul>
                 </li>