Add documentation on how to detect the FrankenPHP version and the Caddy version underneath

[Marian-Kechlibar]

Describe you feature request

I would like to display in my app's About page what is the precise version of FrankenPHP and Caddy running underneath. I am using one of the official Docker images as a base.

The best would be to describe in the documentation, which environment variables or functions can be used to obtain this version information.

[withinboredom]

See #1225

[samdark]

I don't think it exposes this info. There's SERVER_SOFTWARE but it says simply "FrankenPHP". So the following should do the job:

$frankenString = exec('frankenphp -v');
// in my case it is:
// FrankenPHP (WolfiPHP) 1.3.6 Caddy v2.8.4 => /usr/src/caddy@(devel)

[withinboredom]

Correct. It currently doesn't expose the version but the issue I linked above is also linked to an open PR to expose that version string.