doc.wikimedia.org: Create tiled wmui layout, add captions, link repos

The homepage now features a wmui-based tile grid with short
descriptions and links to view the source code.

All existing links are maintained, as well as a few new ones:

• Added missing links for things projects already published. For example Minerva, MobileFrontend, and Popups have Storybooks published but where not yet linked.
• Added a few links to packages (composer, npm, debian).

This layout was previously experimented with at
https://www.mediawiki.org/?title=Wikimedia_Open_Source&oldid=3803609
and https://www.mediawiki.org/?title=Template:OSProject&oldid=3800396.

An internal dump of just CI-published doc directories remains
accessible via https://doc.wikimedia.org/index/. The one or two projects
that use WMF Gerrit+CI for hosting, are not WMF-deployed, but do have
CI-published docs, are now only linked from /index/.

Change-Id: I7f897d9a0ff080f58c0b171f9014695067427a6a