Add static site data export script

bin/export.php connects to the BNETDocs MariaDB database, queries all
published packets, documents, and news posts, pre-renders Markdown to
HTML via Parsedown, and writes the JSON data tree consumed by the
bnetdocs-static GitHub Pages site.

Usage: php bin/export.php --output-dir /path/to/bnetdocs-static/data

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

Author: carlbennett