[docs] Document env substitution using talenv(.sops).yaml

[PrivatePuffin]

I was looking into the correct structure of talenv, besides the fact it isn't that hard, I noticed that it also wasn't actually documented at-all.

It might be best to start documenting things like this.

[budimanjojo]

@Ornias1993 It's like a dotenv file that everything you put there will be loaded as environment variable on runtime so you can do envsubst on talconfig.yaml and talsecret.yaml when running talhelper command. It was initially made for secrets but then the upstream implemented secret file so I added talsecret.yaml to replace it but there are still people using it for something else (like https://budimanjojo.github.io/talhelper/latest/guides/#using-doppler-instead-of-sops) so I leave it there. I'll see if I slip something into a doc later.

[PrivatePuffin]

does talsecret.yaml also get processed into env substitution?

[budimanjojo]

yes, after this commit: b782434

But it's more for people not using sops because talsecret.yaml is only used for secrets.