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 #31916550 | 📁 Comparing aceedc1 against latest (6c48a0b)

🔍 Preview build

Show files changed (17 files in total): 📝 17 modified | ➕ 0 added | ➖ 0 deleted

File	Status
glossary.html	📝 modified
api/generated/pymc.draw.html	📝 modified
api/generated/pymc.icdf.html	📝 modified
api/generated/pymc.logcdf.html	📝 modified
api/generated/pymc.logp.html	📝 modified
api/generated/pymc.sample_posterior_predictive.html	📝 modified
api/generated/pymc.sample_prior_predictive.html	📝 modified
_modules/pymc/distributions/continuous.html	📝 modified
_modules/pymc/distributions/distribution.html	📝 modified
_modules/pymc/logprob/basic.html	📝 modified
_modules/pymc/model/core.html	📝 modified
_modules/pymc/sampling/forward.html	📝 modified
_modules/pymc/smc/sampling.html	📝 modified
api/dims/generated/pymc.dims.HalfFlat.html	📝 modified
api/distributions/generated/pymc.HalfFlat.html	📝 modified
api/distributions/generated/classmethods/pymc.SymbolicRandomVariable.update.html	📝 modified
api/model/generated/classmethods/pymc.model.core.Model.compile_fn.html	📝 modified

[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)