Link to ruby gems documentation

Create a new subsection for the ruby gems and point to the two we are
generating documentation for:

https://doc.wikimedia.org/rubygems/mediawiki-ruby-api/
https://doc.wikimedia.org/rubygems/mediawiki-selenium/

Change-Id: I5d2d7d1948a91dbf33b9be9415cf878917cd268d