Add profiling.sampling documentation

The sampling profiler is new in Python 3.15 and works fundamentally differently
from the tracing profiler. It observes programs from outside by periodically
capturing stack snapshots, which means zero overhead on the profiled code. This
makes it practical for production use where you can attach to live servers.

The docs explain the key concepts (statistical vs deterministic profiling),
provide quick examples upfront, document all output formats (pstats, flamegraph,
gecko, heatmap), and cover the live TUI mode. The defaults table helps users
understand what happens without any flags.

Author: pablogsal