Add docs

[cnpryer]

Closes #79

• mkdocs
• spell check gha

Disclaimer: I don't want to create two places to maintain documentation.

I'm new to mkdocs, but here's what I did to publish an example here https://cnpryer.github.io/rye/

❯ mkdocs build
❯ mkdocs gh-deploy

From https://www.mkdocs.org/user-guide/deploying-your-docs/.

Steps

• [x] Deploy an example
• [x] Finalize user guide