doc: split architecture.rst into a sectioned directory

The 76 KB single file becomes doc/architecture/ with one page per
major topic and an index carrying the intro and stack diagram. All
seven existing ref labels move with their headings, so :ref: links
keep working. Cross-section references that used implicit targets
now use explicit labels, relative link targets gained a directory
level, and the inbound :doc: and anchor links across the tree point
at the new subpages. Since docs.ceph.com serves directory URLs, the
top-level /architecture/ URL is unchanged.

Fixes: https://tracker.ceph.com/issues/77207

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