doc/radosgw: Use ref for hyperlinking to multisite

doc/radosgw: Use ref for hyperlinking to multisite

Use validated ":ref:" hyperlinks instead of "external links" in "target
definitions" when linking within the Ceph docs:
- Update to use existing label in multisite.rst.
- Remove unused "target definitions".

Also use existing label for linking from multisite.rst.
Fix a broken link within multisite.rst.

The rendered PR should look the same as the old docs, only differing in
the source RST.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>