Write the "Why MessageFormat 2?" page

[lucacasonato]

https://messageformat.dev/docs/why/ is currently empty.

This page should help users answer "Why should I use this over MF1 or another message formatting language?":

• rationale as to why we needed a new standard
• what inspiration came from other modern standards like Fluent (@eemeli)
• why users should pick MF2 over any other system
  • compatibility (forward looking statements about native support on multiple major platforms: web, android, etc?)
  • modern capability set
  • extensive documentation and clear specification
  • maybe even tooling? (editor integration, data model, playground, etc)

Anyone in particular interested in writing a first draft of this page? I am happy to go over and give feedback (and tell you how compelling the argument is :D). @aphillips or @eemeli perhaps?