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.