Add Matern12 covariance function to docs

[fonnesbeck]

What is this PR about?

This adds the Matern12 GP kernel to the docs by including it in the __all__ list, and also fixes the math.

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 👇

Bugfixes

Closes #6428

[codecov]

Codecov Report

Merging #6429 (2aee578) into main (f6a502a) will increase coverage by 8.68%.
The diff coverage is n/a.

❗ Current head 2aee578 differs from pull request most recent head f01822d. Consider uploading reports for the commit f01822d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6429      +/-   ##
==========================================
+ Coverage   86.05%   94.74%   +8.68%     
==========================================
  Files         148      148              
  Lines       27646    27646              
==========================================
+ Hits        23792    26194    +2402     
+ Misses       3854     1452    -2402     

Impacted Files	Coverage Δ
pymc/gp/cov.py	98.09% <ø> (ø)
pymc/sampling/parallel.py	88.42% <0.00%> (+1.05%)	⬆️
pymc/logprob/cumsum.py	100.00% <0.00%> (+3.12%)	⬆️
pymc/tests/logprob/utils.py	50.00% <0.00%> (+3.65%)	⬆️
pymc/logprob/rewriting.py	97.05% <0.00%> (+5.88%)	⬆️
pymc/logprob/abstract.py	97.56% <0.00%> (+6.09%)	⬆️
pymc/logprob/utils.py	100.00% <0.00%> (+13.79%)	⬆️
pymc/logprob/joint_logprob.py	97.01% <0.00%> (+19.40%)	⬆️
pymc/logprob/tensor.py	82.40% <0.00%> (+24.00%)	⬆️
pymc/logprob/transforms.py	96.42% <0.00%> (+28.63%)	⬆️
... and 16 more