Add mediawiki-core-docgen job

Will be used to generate MediaWiki documentation pages using the
mw-docgen.sh (I5dd693c81b) tool which publishes the content to
doc.wikimedia.org/mediawiki-core.

Jenkins will not report anything back to Gerrit for this job.

Also added one to 'test/mediawiki' which will be enabled first
in Zuul.

Change-Id: I67cf7405c562522b95ce216be25c3ad93215e980