Write user guide section for learning-based PEC

[Misty-W]

Start outline of user guide section on learning-based PEC

Description

Please explain the changes you made here.

Checklist

Check off the following once complete (or if not applicable) after opening the PR. The PR will be reviewed once this checklist is complete and all tests are passing.

• [X] I added unit tests for new code.
• [ ] I used type hints in function signatures.
• [ ] I used Google-style docstrings for functions.
• [ ] I updated the documentation where relevant.
• [X] Added myself / the copyright holder to the AUTHORS file

If some items remain, you can mark this a draft pull request.

License

• [X] I license this contribution under the terms of the GNU GPL, version 3 and grant Unitary Fund the right to provide additional permissions as described in section 7 of the GNU GPL, version 3.

Tips

• If the validation check fails:

  1. Run make check-types (from the root directory of the repository) and fix any mypy errors.

  2. Run make check-style and fix any flake8 errors.

  3. Run make format to format your code with the black autoformatter.

  For more information, check the Mitiq style guidelines.

• Write "Fixes #XYZ" in the description if this PR fixes Issue #XYZ.

[github-actions[bot]]

:point_left: Launch a binder notebook on branch unitaryfund/mitiq/1365-write-user-guide-section-for-learning-based-pec

[codecov[bot]]

Codecov Report

Merging #1395 (53af7a2) into master (37fa18b) will increase coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1395      +/-   ##
==========================================
+ Coverage   98.21%   98.23%   +0.01%     
==========================================
  Files          63       63              
  Lines        2921     2948      +27     
==========================================
+ Hits         2869     2896      +27     
  Misses         52       52              

Impacted Files	Coverage Δ
mitiq/ddd/insertion.py	100.00% <0.00%> (ø)
mitiq/pec/representations/learning.py	100.00% <0.00%> (ø)

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[Misty-W]

Something very strange happened when I tried to rebase, starting a new branch.

[Misty-W]

Note to self @Misty-W, add reference to SciPy docs and link in docstrings (or figure out external mapping).

[Misty-W]

Note to self @Misty-W, add reference to SciPy docs ~and link in docstrings (or figure out external mapping).~

Reference added in user guide. Syntax for external mapping in docstrings unclear from Sphinx docs.