Auto-format YAML with prettier and pre-commit

[sgibson91]

Summary

In #2726, there was an issue with the book build due to incorrectly formatted YAML. @aleesteele and I suggested it would be a good idea to enforce YAML formatting since it is a tricky thing to hand-edit and book build failures are not super informative.

List of changes proposed in this PR (pull-request)

• Configured pre-commit
  • This is a tool that can be used locally to perform certain, specified actions just before git creates a commit
  • It can also be used in CI so that, for any open Pull Requests, if the files would have been edited when running pre-commit locally, an extra commit applying these changes would be added to the PR
• Configured prettier, which will be executed by pre-commit and perform the auto-formatting
  • Provided a .prettierignore file to restrict which files will be auto-formatted to only YAML. We can change this file as needed to include other file we would like to be auto-formatted.

This combination of pre-commit and prettier is a very common workflow among open source projects.

What should a reviewer concentrate their feedback on?

There will be three commits on this repo:

• The two original commits I made containing the pre-commit and prettier configuration files described above
  • prettier config commit
  • pre-commit config commit
• An automatically generated commit applying prettier auto-formatting to YAML files, by pre-commit
  • [link TBA]

• Does everything look ok?

Acknowledging contributors

• All contributors to this pull request are already named in the table of contributors in the README file.
• The following people should be added to the table of contributors in the README file:

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[sgibson91]

Pre-commit doesn't auto-add correction commits for prettier and it seems this is intentional (see refs below). So this approach will only work for those using pre-commit locally.

• Hook that updates files pre-commit/pre-commit#532
• yapf -i and pass pre-commit check pre-commit/pre-commit#747
• Automatically stage files changed by hook pre-commit/pre-commit#806
• Add changes generated by pre-commit pre-commit/pre-commit#879