👌 Improve [](#target) resolution warnings

[chrisjsewell]

Previously, if [](#target) matched nothing locally, but multiple items in intersphinx inventories, then no warning would be logged, and the first match silently used.
Resolution of intersphinx now proceeds (if no local matches found) the same as for [](inv:#target), such that a myst.iref_ambiguous warning is emitted in this case (the first match is then still used).

Warnings coming from resolution of [](#target) can also now be "individually" turned off using https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-nitpick_ignore, e.g. nitpick_ignore = [("myst", "target")]

[codecov]

Codecov Report

Base: 89.40% // Head: 89.80% // Increases project coverage by +0.40% 🎉

Coverage data is based on head (9bbe97b) compared to base (2902e07).
Patch coverage: 87.75% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #716      +/-   ##
==========================================
+ Coverage   89.40%   89.80%   +0.40%     
==========================================
  Files          23       23              
  Lines        2868     2883      +15     
==========================================
+ Hits         2564     2589      +25     
+ Misses        304      294      -10     

Flag	Coverage Δ
pytests	89.80% <87.75%> (+0.40%)	⬆️

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Impacted Files	Coverage Δ
myst_parser/mdit_to_docutils/sphinx_.py	97.72% <ø> (ø)
myst_parser/mdit_to_docutils/transforms.py	86.84% <ø> (ø)
myst_parser/sphinx_ext/myst_refs.py	88.76% <87.50%> (+1.10%)	⬆️
myst_parser/mdit_to_docutils/base.py	92.87% <100.00%> (-0.01%)	⬇️
myst_parser/inventory.py	86.88% <0.00%> (+4.09%)	⬆️

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