'Cover' section listing code coverage reports

We finally have MediaWiki core code coverage being published at:
https://integration.wikimedia.org/cover/mediawiki-core/master/php/

This creates a new 'Cover' topsection ands adds a link to
mediawiki/core@master coverage report.

bug: 45594
Change-Id: If3d68f3e4e8bbe7152e7852ed80b6b0c4b8b0848