doc/radosgw: edit "Add/Remove a Key"

doc/radosgw: edit "Add/Remove a Key"

Edit the section "Add/Remove a Key" in doc/radosgw/admin.rst. Each
operation (e.g. "Adding an S3 key pair for a user", "Removing an S3 key
pair for a user") now has its own subsection. This increased granularity
should make it easier in the future to link to each of these specific
operations, if needed.

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit f62e93cbe73cd8f624a6c99497051c1a0aaf3ab6)