WIP: Scriptv2 docs

[macobo]

What's new:

• [x] Script update guide
• [ ] Form submission tracking

What's updated:

• [ ] Rewrite docs/script-extensions.md
• [x] Script feature description docs
• [x] docs/plausible-script.md
• [ ] docs/troubleshoot-integration.md
• [ ] Proxy documentation
• [ ] Integrations documentation

TBD:

• [ ] Images
• [ ] Google tag manager guide
• [ ] NPM library references