DOC: Expand RandomWalk docstrings

[Diegomed11]

Description

This PR addresses a TODO in pymc/distributions/timeseries.py.

I have expanded the documentation for the RandomWalk class, adding:

• Mathematical formulation of the random walk process using LaTeX.
• Detailed parameter descriptions (innovation_dist, steps, etc.).
• Adherence to the NumPy docstring standard.

Related Issue

• Closes #
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.49%. Comparing base (cadb97a) to head (ff9aa69).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7992      +/-   ##
==========================================
+ Coverage   90.22%   91.49%   +1.27%     
==========================================
  Files         116      116              
  Lines       18972    18972              
==========================================
+ Hits        17117    17359     +242     
+ Misses       1855     1613     -242     

Files with missing lines	Coverage Δ
pymc/distributions/timeseries.py	94.62% <ø> (ø)

... and 5 files with indirect coverage changes

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