Docs: Add PostgreSQL and ProxySQL to the introduction

A small task for a new contributor.

This little task I'll do myself as a new contributor.

PMM 2 works with PostgreSQL and ProxySQL, but only MongoDB and MySQL are written in the introduction. 

https://www.percona.com/doc/percona-monitoring-and-management/2.x/index.html

Add PostgreSQL and ProxySQL is good for SEO and first impression readers.