Update the website documentation content

[jacobsimionato]

Should be up to date and consistent with code and project roadmap

• docs – a2ui.org (mkdocs)
  • Overview
  • Comparison
  • Roadmap
  • Contributing
  • Guides
    • quickstart
    • onboard agents with and and

[sethladd]

I'm splitting README from website docs

[zeroasterisk]

Closing — this was a general tracking issue and docs are actively maintained. Current effort is PR #622 (v0.9 spec alignment).

Future doc updates can be tracked via specific issues/PRs.