#2949 docs: add Polish README translation

Description

Adds README.pl.md, a complete Polish translation of README.md, and links to it from all existing README language selectors.

Preserves the source document’s technical details, benchmark values, commands, and configuration examples. No runtime code or defaults are changed.

Validation: checked section completeness, link targets, local file references, JSON example parity, and git diff --check. No backend builds were needed for this documentation-only change.