doc/radosgw: Internal link and single-keystroke improvements

doc/radosgw: Internal link and single-keystroke improvements

Use ref for hyperlink instead of abusing "external links" feature for
intra-docs link in cloud-transition.rst, add a label in
cloud-restore.rst for it.

Use auto-generated link text instead of manually adding that same
section title as link text in cloud-transition.rst.

One list item was missing a colon in s3_objects_dedup.rst.

Add space between number and units in s3_objects_dedup.rst.

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