Add functionality to use docutils specialized role

[chrisjsewell]

See: https://docutils.sourceforge.io/docs/ref/rst/roles.html#specialized-roles

[codecov]

Codecov Report

Merging #189 into master will decrease coverage by 0.02%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
- Coverage   91.86%   91.84%   -0.03%     
==========================================
  Files          10       10              
  Lines        1107     1116       +9     
==========================================
+ Hits         1017     1025       +8     
- Misses         90       91       +1     

Flag	Coverage Δ
#pytests	91.84% <91.66%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
myst_parser/mocking.py	87.08% <91.66%> (+0.07%)	⬆️

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