mgr/dashboard: enable different URL for users of browser to Grafana

mgr/dashboard: enable different URL for users of browser to Grafana

Fixes: https://tracker.ceph.com/issues/47386
Signed-off-by: Patrick Seidensal <pseidensal@suse.com>
(cherry picked from commit 676f82923d875cfe9528a401963c09f29a6de7f4)

Conflicts:
        doc/mgr/dashboard.rst
        src/pybind/mgr/dashboard/controllers/grafana.py

Resolved some trivial conflicts and replaced the heading of the newly
added section in the documentation to another level, as the
documentation between master and octopus differs quite much.

(cherry picked from commit 56f304d31b154d87362620f08046720a3e0f3203)

Conflicts:

Removed cephadm documentation from cherry-picked commit as cephadm is
not part of Ceph Nautilus.

Removed paragraph that mentioned cephadm in dashboard documentation.