chore: Set up linting

[mgax]

Fix https://github.com/ietf-tools/www/issues/379

~This PR should be rebased after the other large in-flight PRs get merged.~ ✅

• Set up black and ruff.
• Set up pre-commit.
• Run pre-commit in the CI to enforce lint rules.
• Fix existing linter errors (will need to be redone on rebase).