mgr/dashboard: Add new landing page component

mgr/dashboard: Add new landing page component

Fixes https://tracker.ceph.com/issues/74409

- flagged by DASHBOARD fetaure flag
- added tests
- added layout for overview page
- dropped using get API which is getting polled freqently for perf

Signed-off-by: Afreen Misbah <afreen@ibm.com>
(cherry picked from commit 9a07fbf6e2848025124458faee6db953f6d91a2b)

 Conflicts:
	src/pybind/mgr/dashboard/frontend/src/app/ceph/dashboard/dashboard.module.ts
	src/pybind/mgr/dashboard/frontend/src/app/ceph/dashboard/dashboard/dashboard.component.html
	src/pybind/mgr/dashboard/frontend/src/app/ceph/dashboard/dashboard/dashboard.component.ts
	src/pybind/mgr/dashboard/frontend/src/app/shared/services/feature-toggles.service.ts