Outline that we generally only need one approval from one Maintainer.

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/42891,
we're changing the docs license to be the same across CE and EE.

With that chance, let's change the Omnibus docs as well to be
consistent.

Over time we should aim to reduce the impact of this, but there will
always be some value in hand-testing packages before merge.

  resolves #2163:
  used `dpkg-query -W gitlab` does not show installed versions,
  using wildcard in package name fixes it

Expanding documentation on how to contribute effectively to GitLab and
how to setup a development environment.

Update contributing guide explaining when it is ok to create an issue and how to get help otherwise.