docs: Add link to public-probability-models website

[matthewfeickert]

Description

Following up on PR #1688, add note in the docs about links to the public-probability-models website that generates a nightly list of all public probability models.

• c.f. https://pyhf.github.io/public-probability-models

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add note that links to the public-probability-models website that
  generates a nightly list of all public probability models.
   - c.f. https://pyhf.github.io/public-probability-models

[codecov]

Codecov Report

Merging #1956 (150ce71) into master (bfd5a2a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1956   +/-   ##
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  Coverage   98.24%   98.24%           
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  Files          68       68           
  Lines        4380     4380           
  Branches      728      728           
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  Hits         4303     4303           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	26.55% <ø> (ø)
doctest	60.54% <ø> (ø)
unittests-3.10	96.14% <ø> (ø)
unittests-3.7	96.12% <ø> (ø)
unittests-3.8	96.16% <ø> (ø)
unittests-3.9	96.18% <ø> (ø)

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[matthewfeickert]

Relevant RTD build: https://pyhf--1956.org.readthedocs.build/en/1956/citations.html#published-statistical-models