docs: automarkup: track failed cross-reference attempts

The automarkup code tries to create a lot of cross-references that don't
exist.  Cross-reference lookups are expensive, especially in later versions
of Sphinx, so there is value in avoiding unnecessary ones.  Remember
attempts that failed and do not retry them.

This improves the htmldocs build time by 5-10% depending on the phase of
the moon and other factors.

Tested-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/20220630163630.714673-2-corbet@lwn.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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