docs: automarkup: Move common logic to add and resolve xref to helper

docs: automarkup: Move common logic to add and resolve xref to helper

Several of the markup functions contain the same code, calling into
sphinx's pending_xref and resolve_xref functions to add and resolve a
cross-reference, with only a few of the parameters changed (domain,
reference type, markup content). Move this logic to its own function and
reuse it in the markup functions.

No functional change.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250408-automarkup-resolve-xref-helper-v2-1-e0a9b8fc7fdd@collabora.com