doc: Use existing labels and ref for hyperlinks in architecture.rst

doc: Use existing labels and ref for hyperlinks in architecture.rst

Use validated ":ref:" hyperlinks instead of "external links" in "target
definitions" when linking within the Ceph docs:
- Update to use existing labels when linkin from architecture.rst.
- Remove unused "target definitions".

Also use title case for section titles in
doc/start/hardware-recommendations.rst because change to use link text
generated from section title.

Other than generated link texts 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>