Add the PostgreSQL guide to the index page [ci skip]

Hello,

This is just a tiny pull request that adds the PostgreSQL guide to the index page of guides.rubyonrails.org. It is currently located under the "Digging deeper" part and not the "Models" one as this is a specific use case but let me know if you find it more logical to move it.

By the way, really awesome work @senny! :heart:

Have a nice day.