docs: restore reference-manual README

Bring README.md back to a section-by-section reference walkthrough
(25 H2 sections, 52 H3 subsections, ~1820 lines) covering the current
API surface: full create_webserver builder reference, http_resource
virtuals, the three routing families (on_* / route() / register_path
& register_prefix), full http_request and http_response references,
authentication (Basic / Digest / centralized / mTLS / SNI / TLS-PSK),
WebSocket, daemon introspection and external event loops, threading
contract (DR-008 / §5.1), error propagation (DR-009 / §5.2), and
feature availability.

Examples are linked rather than inlined: the grouped index in the
README mirrors examples/README.md.

examples/hello_world.cpp: drop the PRD §3.4 callout from the file
header so the comment stays self-contained; the README block remains
byte-for-byte synced via scripts/check-readme.sh.

scripts/check-readme.sh and scripts/check-examples.sh both pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: etr