[docs] Rill Developer: Templating and environment variables: unclear how to use .env

[shiv-io]

Describe the bug
Having gone through https://docs.rilldata.com/build/credentials/ and https://docs.rilldata.com/deploy/templating, it is still quite unclear how to use a .env file to define an environment variable and to then use that variable in a source.yaml. I've tried several guesses to make this work but haven't been able to. In particular, I am looking to define a .env with my postgres connection string and use that variable in my source.yaml.

To Reproduce
Try to use a .env file to define an environment variable and then reference that variable in a postgres connection URL.

Expected behavior
It would help to have examples on the docs for this, as .env files are often used by users.

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Additional context
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[royendo]

Hey Shiv!
Thanks for the feedback!

Here's a quick example of using the environmental variables for a source:

type: source

connector: "postgres"
sql: |
  select * from {{.env.table}}

dsn: '{{.env.db_type}}://...'

where the .env is defined as

table="rna"
db_type="postgres"

I'm actually working on making our docs more friendly that would add a templating and project defaults to each object type in Rill (source, model, metrics...) Sounds like this is a timely effort given the feedback :)