See gitlab#66 (comment 2725214)

This still needs work.

This is really about trac more than gitlab, after all. Now that we
migrated, those docs are not really useful anymore and can be archived
to that other page.

I feel the GitLab migration documentation belongs there, and could be
helpful to not pretend Trac never existed...

This reverts commit:

6a447183 remove link to dead trac page
1735010a remove trac documentation now that it is retired