Pre commit

[jacobtomlinson]

I noticed while reviewing #130 that there were some markdown formatting issues. I ran prettier locally and addressed issues raised by my editor (which appears to use markdownlint under the hood).

To make this a little easier to keep on top of this PR adds pre-commit checks that run prettier and markdownlint. I've also added a CI step cribbed from dask/dask to check this automatically.

This PR contains a lot of changes, similar to project wide Black PRs. A summary of changes include:

• Add a pre-commit config file
• Add .prettierrc and .prettierignore config files
• Add a .markdownlint.json config file
• Add a GitHub Action to run pre-commit in CI
• Run prettier and markdownlint on all files.

[jsignell]

Thanks for doing this @jacobtomlinson! I just have one little question