Add workflows for testing, building, and deploying gh-pages

[ghost]

Resolves #43

Adds the following workflows:

• Tests
  • Runs on pushes to main and pull requests against main
  • Lints the codebase
  • Runs i18n.py validate to make sure all locales/languages have complete translations
• Docs
  • Runs on pushes to main
  • Runs mkdocs build and now deploys the built site in the gh-pages branch
• Build
  • Runs on pushes to main and pull requests against main
  • Runs a full docker build to generate firmware
  • This is mostly a sanity check to make sure firmware builds don't get broken somehow

The existing mkdocs folder (containing source files) has been renamed to docs, and the built docs site is no longer in main