docs: add guidelines about environment variables

[alexandrebrg]

Add a brief note about configuration overrides through environment variables.

Description

In version v0.1.6, a feature created the ability to override configuration files options through environment variables. It's something very useful to know when deploying OpenFGA, and there isn't any reference yet in the documentation about it.

This PR adds a simple note in the configuration section to tell people that it's possible to use environment variables.

References

There is no references.

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[CLAassistant]

All committers have signed the CLA.

[rhamzeh]

Added a ticket of what we would like to do: #267

But until we get there, your PR adds a lot of value to anyone reading the docs, thanks @alexandrebrg !