doc/radosgw: Use ref for hyperlinks, 2nd batch

doc/radosgw: Use ref for hyperlinks, 2nd batch

Use validated ":ref:" hyperlinks instead of "external links" in "target
definitions" when linking within the Ceph docs:
- Add a label at beginning of referenced files if missing.
- Remove unused "target definitions".
- Updated links targeting files: compression encryption keystone

Cleaned hyperlinks usage in kmip.rst:
- Some links were using anonymous links (double underscore) unnecessarily.
- Some links were not using backticks, add for consistency.
- Move anonymous link definition to after the ordered list to avoid
  unnecessary empty line between list items.

Use an already existing label for 2 intra-docs links that used full URLs.
Use an already existing label for intra-docs link instead of a file name
reference in s3/authentication.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>