Create docs page explaining importance of github.com token for Mend apps

[rarkins]

Describe the proposed change(s).

Add a new page to Renovate docs which explains why a github.com token is needed when running on Bitbucket Cloud or AZDO, with clear instructions on how to add one. This overlaps a little with the existing docs on secrets but I don't think the existing guide is clear enough to link to alone. Maybe the part on "adding a host rule through the UI" could be split off into a separate page even. It just needs to be clear and not confusing.

Also state clearly what happens if you don't have such a token (it's ok, but there are limitations)