docs: Improve Database SSL option section

[ivarconr]

Today we get a few question every now and then about SSL options for the database connection.

1. Typical Postgres "sslmode" options not supported.

Unleash builds directly on node-postgres and we sometimes get questions if Unleash could support specific sslmodes. The underlying library does no longer support all the sslmodes (announcement) and Unleash cannot support all of them for this reason.

The most asked about is "sslmode=prefer" (eg. #1138), and we can simply not support that as long as the driver does not do that. You need to know upfront if you require a SSL connection or not.

2. DATABASE_SSL environment variable

We are simply passing the ssl options directly to node-postgres. To make it more convenient for the user we also allow the DATABASE_SSL to be written as JSON and will parse that variable on startup and pass the result to the node-postgres library. We should create some real examples on how this looks like, to make people less confused (e.g #1432)