ci: Deploy GitHub Pages docs directly using GitHub Actions

[matthewfeickert]

Description

Following the blog announcement GitHub Pages: Custom GitHub Actions Workflows (beta), publish the GitHub Pages docs directly from the repository using GitHub Actions, without needing to set up a publishing source like a branch.

This also restricts deployments to happening in specific environments (c.f. https://github.com/scikit-hep/pyhf/settings/environments).

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

* Use the GitHub Actions:
   - actions/configure-pages@v2
   - actions/upload-pages-artifact@v1
   - actions/deploy-pages@v1
  over the peaceiris/actions-gh-pages@v3 to deploy the GitHub Pages docs website.
  This additionally uses the `permissions` and `environment` settings to restrict
  the deployment to specific environments.
   - c.f. https://github.blog/changelog/2022-07-27-github-pages-custom-github-actions-workflows-beta/
* Remove unused strategy matrix as only one Python version is ever setup.
* Remove creation of `.nojekyll` file as the website is created without the
  use of Jekyll (Jekyll was the previous default assumption from GitHub pages).

[codecov]

Codecov Report

Merging #1955 (e73b7f3) into master (82bd0f7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1955   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files          69       69           
  Lines        4439     4439           
  Branches      748      748           
=======================================
  Hits         4362     4362           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	25.81% <ø> (ø)
doctest	61.02% <ø> (ø)
unittests-3.10	96.14% <ø> (ø)
unittests-3.7	96.13% <ø> (ø)
unittests-3.8	96.17% <ø> (ø)
unittests-3.9	96.19% <ø> (ø)

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