Adds shape/rate info to Gamma docs

[Dpananos]

Description

I've had to keep going back and forth to the Wiki entry for the Gamma distribution to remind myself how the gamma is parameterized. This PR adds a line to let readers know this is the shape and rate parameterization.

This is what the change looks like

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Related Issue

NA

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
• [ x] Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pymc--7625.org.readthedocs.build/en/7625/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.83%. Comparing base (5d51953) to head (4bf8589).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7625   +/-   ##
=======================================
  Coverage   92.83%   92.83%           
=======================================
  Files         106      106           
  Lines       17748    17748           
=======================================
  Hits        16477    16477           
  Misses       1271     1271           

Files with missing lines	Coverage Δ
pymc/distributions/continuous.py	98.25% <ø> (ø)