doc/radosgw: change all intra-docs links to use ref (3 of 6)

Part 3 of 6 to make backporting easier. Depends on part 1.

Use the the ref role for all remaining links in doc/radosgw/ with the
exception of config-ref.rst which will depend on changes to rgw.yaml.in.

The "external link definitions" syntax being removed is intended for
linking to external websites and not for intra-docs links. Validity of
ref links will be checked during the docs build process.

Add labels for links targets if necessary.
Remove unused external link definitions in the modified files.

Use confval instead of literal text for 2 configuration keys in
vault.rst.

Signed-off-by: Ville Ojamo <git2233+ceph@ojamo.eu>