No more Ruby dependency and most important more than 10 times less build time (at least on Windows)!

Much lighter, more modern and cleaner codebase.

* Prep for v4.3.0

* More updates for v4.3.0.

* rerun to build docs source map

- Move 4.1 docs to 4.2
- Update versions everywhere to 4.1.3 with release script
- Manually bump the shorthand version in package.json
- Add 4.2 to the versions docs page
- Update some redirects
- Fix tests asset URLs
- Bump Nuget and more

We use the official HTML validator for HTML validation, and we don't really use any of the htmllint features.

We can achieve the same very easily.