Rewrite the README as a lean intro and move detail into docs/

The README had grown to ~360 lines dominated by an exhaustive list of
edge-case divergences from sqlite3, which buries a first-time visitor.
Replace it with a short overview: what the driver is, sync and async
examples, the few key facts (paramstyle, autocommit-by-default,
SERIALIZABLE isolation), and the four-package dqlite stack with links.

Move the detail into docs/: the transaction model (autocommit by default,
how to group statements, isolation_level, leader-flip-during-commit), a
curated set of differences from sqlite3 (with the niche edge cases kept
but grouped separately), and the differences from aiosqlite.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: antoineleclair