Add documentation

There is **0** documentation, in a GUI software maybe dialing extensions is not hard to do without a user guide, but in a command line client users expect at least a `--help` parameter.

I think a collection of `.rst` files for the list of topics bellow will do:
- Install
- Configuration
- Dialing extensions

Later I can add sphinx support, but right now these 3 topics require proper documentation.

Start by reviewing the command-line client and its existing `--help` behavior to understand the user-facing workflow. Create `.rst` documentation covering Install, Configuration, and Dialing extensions. Done means all three topics have clear user guidance, with no Sphinx setup required yet.

Written by the indexing model from the issue text.