Update to include docker files for building Gerrit website

This change adds three files. With these files, Gerrit users
(with appropriate permissions) can build the new Jekyll-based
Gerrit website. These files require that you install:

* docker
* docker-compose

With those tools installed, build the site using the following command
from the jekyll-source directory:

docker-compose up

The generated files appear in the jekyll-source/_site directory.

In addition to these changes, I fixed a minor issue with some index.md
files.

Change-Id: I9137880b51ee5282267f0c6f29cbb3c539822cae