doc/rados: document require_osd_release

doc/rados: document require_osd_release

Add a standalone concept page for the OSDMap require_osd_release field,
the upgrade-gate counterpart to require_min_compat_client. Cover:

 - how to set it and how to check it;
 - the full set of pre-commit guards the monitor runs, rendered as a
   table with each guard's error text and bypass status;
 - which commands and features become available as the flag is raised,
   per release;
 - the OSD boot window that refuses OSDs more than two releases ahead
   of the flag;
 - the OSD_UPGRADE_FINISHED health warning that prompts admins to set
   the flag after an upgrade;
 - the initial value on new clusters and the two mon_debug_* knobs
   that override it for testing.

Also cross-link the new page from the related-flags table on
require-min-compat-client.rst, and from the rados operations index.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>