update docs for paraglide 2.0

[samuelstroschein]

Context

The docs need to be updated for Paraglide JS 2.0.

Topics

• [ ] fallback feature of using locales like en and en-US
• [ ] what changed in v2
• [ ] three ways to compile
  • [ ] cli
  • [ ] bundler plugin
  • [ ] programatically as library PARJS-211
• [ ] variants
  • [ ] gender
  • [ ] plural
  • [ ] a/b testing
• [ ] update architecture diagram to use locale instead of languageTag
• [ ] incrementally migrating to paraglide js
  • [ ] paraglide js can be used simultaneously to every inlang plugin (i18next, next intl, etc)