Strip addon-name parentheticals from addons docs side nav

The right-side table of contents on https://miren.md/addons listed each Environment Variables subsection with the addon name in parentheses (e.g. "PostgreSQL (miren-postgresql)"), which cluttered the nav since the TOC is generated from heading text.

Removed the parentheticals from the five Environment Variables subsection headings in docs/docs/addons.md:

• PostgreSQL (miren-postgresql) → PostgreSQL
• MySQL (miren-mysql) → MySQL
• Valkey (miren-valkey) → Valkey
• RabbitMQ (miren-rabbitmq) → RabbitMQ
• Memcached (miren-memcache) → Memcached

The addon names remain documented in the "Available Addons" table at the top of the page.