The upstream repo (and the release downloads)
github.com/rundocs/jekyll-rtd-theme has been deleted. This broke our
docs generation as the remote theme configuration depended on
downloading the release artefact.
This patch changes the docs building to use a Ruby gem instead of the
remote theme setting. To complicate matters, the gem has an seemingly
incorrect (too strict) version dependency. To mitigate this, we now
install bundler-override plugin to ignore this particular dependency.
The netlify conf is a hack, but I wasn't able to figure out a way how to
install the bundler-override plugin without doing all ruby
initialization in the build command.
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.
