doc: improve HalfFlat class docstring

[KRYSTALM7]

Improves the class-level docstring for the HalfFlat distribution in pymc/distributions/continuous.py.

Changes:

• Expanded the one-line docstring into a full numpydoc-compliant docstring
• Added extended summary describing the improper prior nature of the distribution
• Added PDF formula using proportional notation
• Added support and moments table (Mean and Variance are both undefined)
• Added usage example
• No Parameters section (distribution takes no parameters)

Related to #5459
Closes #8208

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[read-the-docs-community]

Documentation build overview

📚 pymc | 🛠️ Build #32272942 | 📁 Comparing 4c6292a against latest (4b0b6ee)

  🔍 Preview build  

217 files changed · + 4 added · ± 213 modified

+ Added

• api/generated/classmethods/pymc.ode.DifferentialEquation.pullback.html
• api/generated/classmethods/pymc.ode.DifferentialEquation.pushforward.html
• api/distributions/generated/classmethods/pymc.SymbolicRandomVariable.pullback.html
• api/distributions/generated/classmethods/pymc.SymbolicRandomVariable.pushforward.html

± Modified

• genindex.html
• glossary.html
• api/backends.html
• api/logprob.html
• contributing/developer_guide.html
• contributing/jupyter_style.html
• _modules/pymc/data.html
• _modules/pymc/pytensorf.html
• _modules/pymc/testing.html
• api/distributions/multivariate.html
• and 203 more...

[OriolAbril]

This is looking much better now, thanks!

If you haven't yet, you can check the preview at https://pymcio--8211.org.readthedocs.build/projects/docs/en/8211/api/distributions/generated/pymc.HalfFlat.html (from the readthedocs comment "Preview build" then browse to the respective page)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (v6@0b6fc6a). Learn more about missing BASE report.

Additional details and impacted files

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  Files         ?      124           
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  Misses        ?     1653           
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Files with missing lines	Coverage Δ
pymc/distributions/continuous.py	98.27% <ø> (ø)

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