[documentation / submitty.org] Containerize documentation site

[Pavan-Srivathsa]

Why is this Change Important & Necessary?

Building the documentation site requires installing Ruby and managing
gem dependencies locally, which is painful for infrequent contributors
(as noted in Submitty/Submitty#12685). Ruby version conflicts and native
extension compilation failures are common pain points.

What is the New Behavior?

Contributors can now run the documentation site with a single command:
docker compose up

No Ruby, Bundler, or gem installation required. The setup includes:

• Volume mounts for live reload (edits show instantly)
• A named volume for gem caching (fast restarts)
• Force polling for cross-platform file change detection
• LiveReload for automatic browser refresh

What steps should a reviewer take to test?

1. Install Docker if not already installed
2. Clone this branch
3. Run docker compose up --build
4. Verify the site loads at http://localhost:4000
5. Edit any markdown file in _docs/ and confirm the browser refreshes
6. Run docker compose down && docker compose up to confirm fast restart
7. Optionally test the link checker:
   docker compose exec docs bundle exec jekyll build
   docker compose exec docs bundle exec htmlproofer ./_site --assume-extension --empty-alt-ignore --disable_external

Automated Testing & Documentation

README.md updated with Docker setup instructions. Existing Ruby/Bundler
instructions preserved. No automated test changes needed.

Other information

Not a breaking change. No migrations required.
Resolves Submitty/Submitty#12685