Give the README a clearer technical story

[Astn]

Summary

• Lead with a runnable JSON-RPC exchange, then explain the performance story from direct calls through async work, Kestrel and framework comparisons.
• Keep the published benchmark tables, figures and measured numbers intact, with the conditions and comparison caveats beside their results.
• Move detailed binding, hosting, error, session, configuration and security behavior into a new API reference page linked from the Overview and site navigation.
• Render the README's expandable benchmark notes correctly on the Typst site and update cross-document links.

Verification

• python benchmarks/charts/render.py --check
• python -m unittest benchmarks/charts/test_render.py (43 tests)
• python site/build.py --out site/_out
• git diff --check