Slim down README, remove docs now in Sphinx site

README trimmed to intro + quick example + links to full docs at
bluedynamics.github.io/zodb-json-codec/. Removed TYPE_MAPPING.md,
BENCHMARKS.md, and PERF_REPORT_*.md — all content is now in the
Sphinx documentation (reference, explanation sections).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jensens