update Exponential numpydoc

[chritter]

Addresses #5459.

Ensures pm.Exponential class descriptions and docstrings are conform with numpy doc validation.

Note we introduced restructured text syntax for a link (aeppl.logprob <https://github.com/aesara-devs/aeppl/blob/main/aeppl/logprob.py/>_.) which I would propose to adopt for all code. What do you think?

Depending on what your PR does, here are a few things you might want to address in the description:

• what are the (breaking) changes that this PR makes?
• important background, or details about the implementation
• are the changes—especially new features—covered by tests and docstrings?
• linting/style checks have been run
• consider adding/updating relevant example notebooks
• right before it's ready to merge, mention the PR in the RELEASE-NOTES.md

#DataUmbrellaPyMCSprint

Working with @jmloyola.

[codecov]

Codecov Report

Merging #5491 (6d88a59) into main (d150d55) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #5491      +/-   ##
==========================================
- Coverage   87.61%   87.59%   -0.03%     
==========================================
  Files          76       76              
  Lines       13698    13698              
==========================================
- Hits        12002    11999       -3     
- Misses       1696     1699       +3     

Impacted Files	Coverage Δ
pymc/distributions/continuous.py	97.23% <ø> (ø)
pymc/parallel_sampling.py	86.71% <0.00%> (-1.00%)	⬇️