Allow plugins to provide an 'about' section on their documentation index

If a plugin provides a documentation file named "about.md|html", its
content will be inserted at the top of the index page above the lists
of documentation and commands.

If both "about.md" and "about.html" exist, only the first one to be
discovered from the JAR file will be used.

Change-Id: Id0de957b4cc5c2763e945310e252b59e27756639
2 files changed