Feature Request: document `cat` and `credential` template functions

[Skyb0rg007]

Currently one can define secrets in environment variables and use {{ getenv("MYSECRET") | js }} to interpolate the value.
Some environments, for example systemd credentials, encourage passing secrets in files.
I would like to use {{ readFile(getenv("CREDENTIALS_DIRECTORY") + "/mysecret") | js }} to interpolate the secret value into the configuration file.

Authelia has the secret template function which could give inspiration.

[Skyb0rg007]

It looks like this is already a feature!

I guess I’ll change my issue to “Document cat and credential template functions”

[digvijay-y]

I up for documenting!