Retire searchlight repos: update doc site
Retire searchlight repos: update doc site

Searchlight project is retiring in Wallaby cycle[1].
This commit update openstack-manual to redirect
the repo doc site to REAMDE file.

Needed-by: https://review.opendev.org/c/openstack/python-searchlightclient/+/764529
Needed-by: https://review.opendev.org/c/openstack/searchlight-ui/+/764528
Needed-By: https://review.opendev.org/c/openstack/searchlight-specs/+/764527

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-November/018637.html

Change-Id: I9cf18a7b383810f54a0e00af3ebb78d18d495297

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals