Remove the link to the jenkins documentation build.

This link has been broken since the mf4 server crash,
and we're not planning to reinstate the jenkins service.
Instead this should be replaced with something built
by gitlab's ci.
1 job for master in 1 minute and 4 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #10495
docker
build

00:01:04