doc/install: fix empty Upgrade toctree and tab indentation

The Upgrade Software section ended with a toctree that had no
entries, so nothing rendered. Point at the release notes and the
cephadm upgrade guide instead, and normalize the tab-indented
toctree entries to spaces.

Fixes: https://tracker.ceph.com/issues/79042

Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>