[Documentation] Add example for accessing private Git Repositories

[jadczak]

Zero to BinderHub Section 3.7.2 Accessing private repositories contains details for adding tokens for private GitHub and GitLab repositories, but does not discuss configuring access to a private Git repository (e.g. ssh://git@myserver:/srv/git/my-repo.git).

Proposed change

Add documentation / clarify configuring BinderHub for accessing arbitrary private Git repositories.

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