Add documentation for github private repos

[AndyZe]

I got the idea from here: https://stackoverflow.com/a/72883304/3499467

[AndyZe]

Pinging for review. I think this is a small, straight forward change that will save people hours of research.

[130s]

I'm a weak user of Github Action and reviewed from that perspective, can't verify the legitimacy.

[130s]

Thanks for iterating @AndyZe.

@mathias-luedtke Care to review?

[AndyZe]

Ping

[mathias-luedtke]

This does not enable access to private repositories (like the Gitlab version), but clones selected repositories into the current repositories (which might lead to nested packages).

These shortcomings must be stated with a warning!
Or (preferred) we implement support for SSH secrets, like we do for Gitlab.

[mathias-luedtke]

#902 adds support for a single deploy key.
#815 does something similar (but is it not supported yet).

For accessing Github (from a GHA), it might be simple to use a PAT anyway.

[mathias-luedtke]

@AndyZe: #902 has feature parity with the gitlab implemantation.

For multiple deploy keys, webfactory/ssh-agent could be used as well:
https://github.com/mathias-luedtke/sshtest/blob/main/.github/workflows/main.yml#L40, but it seems to have an issue during the cleanup (webfactory/ssh-agent#184).