doc: add exception handling mechanism and CRuby contrast (#897)

Document monoruby's raise/unwind/rescue pipeline and how it differs from
CRuby, centered on the laziness that keeps the raise path cheap:

- MonorubyErr (in-flight Rust error) vs the lazily-materialized Ruby
  exception object (take_ex_obj).
- The two families of MonorubyErrKind: real exceptions vs control-flow
  pseudo-exceptions (return/break/throw/retry/redo/fatal), and why the
  latter are dispatched before any trace capture.
- handle_error as the per-frame unwinder, the per-method exception table,
  ensure deferral, and $! restoration.
- Backtrace construction: incremental capture on unwind, the catch-time
  caller walk (complete_backtrace_for_rescue) as the last coherent
  snapshot point, lazy string formatting + memoization, and why loop's
  StopIteration pays almost nothing.
- Point-by-point CRuby contrast (eager-at-raise snapshot vs
  deferred/incremental) and a hot-path cost table.

Captures the findings from the Exception#backtrace work (#896).


Claude-Session: https://claude.ai/code/session_01XpzdoFu46d2ChJpSzeWsNK

Co-authored-by: Claude <noreply@anthropic.com>

Author: sisshiki1969