Add documentation site

[leslieyip02]

Currently, there are 2 sources of documentation:

1. The docs/ folder contains markdown files
2. ./gradlew dokkaHtml can be used to generate Kotlin API documentation

This PR combines both into a static site, which can be deployed via GitHub Pages.

Static Site Generation

I added MkDocs with Material for MkDocs to generate static HTML content from the markdown in the docs/ folder.
The commands are all grouped together, and serving the site locally is as simple as:

# Assuming you have python3 installed somewhere
./scripts/build_docs.sh
./scripts/serve_docs.sh

The end result looks like this:

The Kotlin API docs can also be accessed at /api:

Note: I moved the docs/Module.md file used by Dokka into dokka/module.md so that docs/ is purely MkDocs content.

I feel that having a static site improves the docs reading experience, and also allows more customization. For example, we can add notes, warnings, TODOs, and collapsible sections.

GitHub Actions

I've added a workflow to host the docs site on GitHub Pages. To set this up, go to Settings > Pages and set the "Build and deployment" source to GitHub Actions.

Since I don't have repo permissions, I created a fork to test the GitHub Actions as well: https://leslieyip02.github.io/jade/ (the content here is different, I just deployed it test the workflow).

Content Cleanups

I tried to clean up the docs as much as I can, and I grouped them as follows:

nav:
  - Home: index.md
  - User Guide:
      - Overview: user-guide/overview.md
      - Commands: user-guide/commands.md
  - Developer Guide:
      - Overview: developer-guide/overview.md
      - Algorithms: developer-guide/algorithms.md
      - Commands: developer-guide/commands.md
      - Style Guide: developer-guide/style.md
  - Testing:
      - Overview: testing/overview.md
      - Maven Development: testing/maven.md
  - Internal:
      - Roadmap: internal/roadmap.md
      - Notes: internal/notes.md
      - Links: internal/links.md
      - Archive: internal/archive.md

That said, there's a lot of confusing / outdated docs so I dumped everything I wasn't sure about into archive.md.