Enable ease of development and deployment

[yiliansource]

In order to allow developers to easily work on the site and deploy it subsequently, the following changes should be established:

• Remove the built files from the main branch: Since they are compiled from source, we should never need them on the main source branch.
• Change the codebase to TypeScript + Sass: This is to allow type-safe development of custom frontend code on the site, as well as faster development of web styles.
• Setup auto-formatting using Prettier: This is to establish and enforce a concistent code-style in JavaScript/TypeScript/Markdown/etc. files.
• Setup linting using ESLint: Similarly to Prettier, this is used to establish consistent code-style and catch common pitfalls.
• Setup automatic deployment using GitHub Actions: This is to avoid having to manually build the site on every commit to main. The action runner should compile the site from source and push the resulting files onto the gh-pages branch.