Setup pre-commit for GEF

[hugsy]

Description/Motivation/Screenshots

This PR adds pre-commit to facilitate and unify the formatting of everything in this project (code, test, docs, ci).

Aside from .pre-commit-config.yml and docs/.markdownlint.yaml, all the other changes are purely cosmetic, applying markdownlint and fixing here and there to make it pass.

It also creates a new CI job that'll simply execute pre-commit for new PRs.

Against which architecture was this tested ?

"Tested" indicates that the PR works and the unit test (see docs/testing.md) run passes without issue.

N/A

Checklist

• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.

[github-actions]

🤖 Coverage Update

• Commit: 175ccf2
• Current Coverage: 71.5684%
• New Coverage: 71.5585%
• Diff: -0.009900000000001796

[github-actions]

🤖 Coverage Update

• Commit: c3130db
• Current Coverage: 71.5585%
• New Coverage: 71.5585%
• Diff: 0.0

To this point, this PR:

• does not include changes to tests
• does not include changes to documentation