Add missing domain methods

[BrennanGit]

This PR adds some missing methods to the new domain introduced in v0.6.0.
We were seeing that not having merge_domaindata implemented was causing our parallel sphinx builds to fail.

note: adding a failing test first to demonstrate the fix works

Resolves #98

[codecov]

Codecov Report

❌ Patch coverage is 75.86207% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.71%. Comparing base (f918d4e) to head (d394791).

Files with missing lines	Patch %	Lines
sphinxarg/ext.py	61.11%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
+ Coverage   50.15%   50.71%   +0.56%     
==========================================
  Files          18       19       +1     
  Lines        1292     1321      +29     
  Branches      262      269       +7     
==========================================
+ Hits          648      670      +22     
- Misses        600      605       +5     
- Partials       44       46       +2     

Flag	Coverage Δ
unittests	50.49% <75.86%> (+0.56%)	⬆️

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[BrennanGit]

Hi, just bumping this as I'm being asked by my team to hard-pin sphinx-argparse to the previous version until its resolved.