feat: Promote modifications to public API

[kratsg]

Pull Request Description

Resolves #1652.

Docs: https://pyhf--2339.org.readthedocs.build/en/2339/_generated/pyhf.pdf.Model.html#pyhf.pdf.Model

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• Tests are passing
• "WIP" removed from the title of the pull request
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* Promote `pyhf.pdf._MainModel._modifications` to `pyhf.pdf._MainModel.modifications`.
* Promote `pyhf.pdf.Model._modifications` to `pyhf.pdf.Model.modifications`.
* Add `pyhf.pdf._MainModel` and `pyhf.pdf._ConstraintModel` to the docs.
* Add documentation for the `modifications()` functions.

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.68% 🎉

Comparison is base (c096b33) 97.59% compared to head (6d6e93a) 98.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2339      +/-   ##
==========================================
+ Coverage   97.59%   98.28%   +0.68%     
==========================================
  Files          69       69              
  Lines        4538     4538              
  Branches      803      803              
==========================================
+ Hits         4429     4460      +31     
+ Misses         65       45      -20     
+ Partials       44       33      -11     

Flag	Coverage Δ
contrib	97.86% <100.00%> (?)
doctest	60.99% <75.00%> (ø)
unittests-3.10	96.29% <100.00%> (ø)
unittests-3.11	96.29% <100.00%> (ø)
unittests-3.8	96.31% <100.00%> (?)
unittests-3.9	96.34% <100.00%> (ø)

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Files Changed	Coverage Δ
src/pyhf/pdf.py	98.03% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[matthewfeickert]

Thanks @kratsg. I just have a docs styling comment, but other than that LGTM.

[matthewfeickert]

(Though maybe we also wait to see if @alexander-held has any comments before merge, given that he opened up Issue #1652)

[alexander-held]

Nothing else from me, thanks for addressing this!

[matthewfeickert]

LGTM. Thanks @kratsg!