Improve the secrets file docs

[spkane]

Adding some more thorough examples since the documentation is sparse here and in the upstream library. I basically had to write a test application and read some various docs about the .env format to figure the logic out.

[spkane]

@ashwilliams1 Is there someone specific who can review this? I don't believe it needs any updating unless someone wants to simply ensure that it is correct.

[bradrydzewski]

we would probably want to document the env file format in a separate, dedicated page, and then link to that page from the quick start. The env file is used in multiple places (not just the drone exec) so having a separate reference document would be preferred. This also keeps are quick starts high level.

[spkane]

That makes sense. I'll leave you all to it then. I just hope that something gets merged in soon-ish, as this took some time to track down and needing to pass in something like an SSH key is a standard enough use case that this should help others avoid the difficulty of figuring this out themselves or just giving up.