Document support for authenticating using environment variables

[mnikolic-ep]

I was originally going to submit an issue requesting support for authenticating to Postgres using environment variables, but I found this comment in a related issue that says this is supported! (I confirmed the listed env vars work).

It would be nice to have this documented in the main provider config page in case other people like me also desire this, but don't bother attempting the default Postgres env vars. 😅