docs: sphinx/automarkup: add cross-references for ABI

docs: sphinx/automarkup: add cross-references for ABI

Now that all ABI files are handled together, we can add a feature
at automarkup for it to generate cross-references for ABI symbols.

The cross-reference logic can produce references for all existing
files, except for README (as this is not parsed).

For symbols, they need to be an exact match of what it is
described at the docs, which is not always true due to wildcards.

If symbols at /sys /proc and /config are identical, a cross-reference
will be used.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/0b97a51b68b1c20127ad4a6a55658557fe0848d0.1739182025.git.mchehab+huawei@kernel.org