Move CustomDist logic to dedicated module and docs section

[markgreene74]

@Hemant19870601 and I worked on this PR during the PyMC Hackathon at PyData London 2024.

Description

With this PR CustomDist is moved to a separate file (custom_distribution.py) along with related classes (CustomDistRV, CustomDistRV, CustomSymbolicDistRV, _CustomSymbolicDist) and helper functions (default_not_implemented, default_support_point).

The imports in distribution.py and __init__.py have been updated to reflect the changes.

DensityDist has been moved to the new file as well.

Note that in distribution.py the import of CustomSymbolicDistRV has been added close to where the class is used to avoid circular import. A noqa has been added on the same line to avoid a ruff error for the misplaced import (E402).

Similarly, the CustomDist tests have been moved to their own file (test_custom_distribution.py) and are passing.

Test results (before/after):	file	before (main branch)	after
test_distribution.py	71 pass/14 ignored/85 total	33 pass/0 ignored/33 total
test_custom_distribution.py	N/A	38 pass/14 ignored/52 total
	71 pass/14 ignored/85 total	71 pass/14 ignored/85 total

Related Issue

• [X] Closes #7247
• [x] Closes #7373
• [ ] Related to #

Checklist

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

Type of change

• [ ] New feature / enhancement
• [ ] Bug fix
• [x] Documentation
• [X] Maintenance
• [X] Other (please specify): Refactor

---

📚 Documentation preview 📚: https://pymc--7363.org.readthedocs.build/en/7363/

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[markgreene74]

Hi @ricardoV94 thank you for all the comments :) I think this PR should be ready for review now, happy for me to change the status?

[ricardoV94]

I rebased the PR on top of main to solve the conflicts with #7370

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Codecov Report

Attention: Patch coverage is 94.59459% with 14 lines in your changes missing coverage. Please review.

Project coverage is 92.19%. Comparing base (e71d1cb) to head (9da36c3).

Additional details and impacted files

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