Default VonMises kappa to 1.0 and fix math notation in doc

[wd60622]

What is this PR about?
This PR fixes the default value concern raised in the issue here. The docs are also adjusted.

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

• None

New features

• VonMises without args will allow sampling

Bugfixes

• pm.draw, pm.sample... will now produce sample with defaults / no args in VonMises distribution

Documentation

• The math will render now. Defaults now updated

Maintenance

• ...

[wd60622]

I didn't see any tests for checking the samples, draws, etc of the VonMises distribution. The tests that I did see provided a kappa all that were not the prior default (None)

If there is a place someone feels appropriate to add an additional test, let me know.

[codecov]

Codecov Report

Merging #6419 (a3ffc56) into main (a079153) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6419   +/-   ##
=======================================
  Coverage   94.73%   94.73%           
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  Files         148      148           
  Lines       27645    27645           
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  Hits        26190    26190           
  Misses       1455     1455           

Impacted Files	Coverage Δ
pymc/distributions/continuous.py	97.68% <100.00%> (ø)

[ricardoV94]

I suggest we don't need a test for this one, I don't think we test default parameters anywhere else

[wd60622]

New docs

[ricardoV94]

Thanks @wd60622 !