Allow navar.md to enable Markdown extensions

A site's pages may require specific Gitiles extensions to Markdown.
For example the Gitiles Markdown documentation uses its extensions to
demo how those extensions are formatted.

Use a named link in navbar.md as metadata to activate extensions that
may have been disabled by the site's configuration, e.g.:

  [home]: /
  [logo]: /images/logo.png
  [extensions]: blocknote, multicolumn

Change-Id: Id43e5d103bbd6b31ebfcb2565c0600d45075bc16

navbar.md

diff --git a/navbar.md b/navbar.md
index 608531c..9f8ac26 100644
--- a/navbar.md
+++ b/navbar.md
@@ -2,3 +2,5 @@
 * [Markdown](/Documentation/markdown.md)
 * [Configuration](/Documentation/config.md)
 * [Developers](/Documentation/developer-guide.md)
+
+[extensions]: blocknote, multicolumn, namedanchor, smartquote, toc