doc: Use validated links with ref instead of external links

doc: Use validated links with ref instead of external links

Use :ref: for intra-docs links that are validated, instead of external
links.
Only use already existing labels.
Fixes a few anchors that pointed to now-renamed section titles.
Use automatically generated link text where appropriate.
Delete unused link definitions.

Mostly in doc/rados/ but also a few in doc/rbd/.
Try to fix all links in each of the changed documents.

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