Make danger pick reviewers and maintainers at random, for feontend,
backend, database, etc, changes, whenever files belonging to those
teams get changed.

By setting expectations both the contributor and reviewer have more
certainty on what gets reviewed when, and at what speed changes could be
merged.

closes #8608

Remove assignee_id=0 condition from search filters, we agreed
that just presence of "ready of review" label is sufficient
for filtering MRs, no need to unassign yourself.

Improve recommended maintainer docs

A section was moved in f644b8c8 but this got lost.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

[ci skip]

We want to describe these roles in a way that is more understandable to
people not familiar with GitLab.

[ci skip]

[ci skip]