chore: switch LibreChat from dev to stable release image

📚 docs: add example configuration files for LibreChat

Verbatim copy of the official LibreChat .env.example from upstream
so users can reference all available environment variables.

Use ghcr.io/danny-avila/librechat:latest (tracks tagged releases
like v0.8.3) instead of the lc-dev image which tracks the main
branch nightly builds.

Verbatim copy from the official LibreChat repo so users can see
all available configuration fields and example values.