README: setup, hosting modes, configuration, benchmarks for 2.0

Rewritten around what a reader needs in order: packages and targets, a
service and the string/byte entry points, the hosting modes (in-process,
Kestrel HTTP, Kestrel raw connections, Blazor WebAssembly, classic ASP.NET),
configuration (serializer resolution and options, MaxDepth, the jsonrpc
version policy, exception details, sessions, hooks), how to run each
benchmark mode with fresh numbers for sync, Task, Kestrel and WebAssembly,
and the 1.x upgrade list. The sample README explains why the typed
[JSExport] row is two orders of magnitude faster and what to do with that.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: Astn