Sync our customizations under _includes/ with v2.0.9 of
jekyll-rtd-theme. Also, pin the theme version in order to avoid abrupt
build failures in the future (caused by our customization becoming
incompatible with the rolling latest version).
Also, drop jekyll-rtd-theme from the Gemfile. It's useless there as it's
being mandated by remote-theme setting in _config.yml.
Add rules for building and serving the html documentation inside a
container.
- 'make site-build' will build the html content unders docs/_site.
- 'make site-serve' builds the site content and serves it at
http://localhost:4000/
