- Rename `coffee setup` command to `coffee link`
- Change function names from `setup_*` to `link_*` (e.g., `setup_with_cln` to `link_with_cln`)
- Update command references in `httpd` and `plugin` crates
- Modify relevant documentation to reflect the command name change

This refactor is to accommodate the addition of a new `unlink` command.

Signed-off-by: Tarek <tareknaser360@gmail.com>

- Add methods in `coffee_core` to enable unlinking coffee configuration from CLN configuration
- Update `coffee_cmd` crate to support the new `unlink` command
- Add documentation for the new `unlink` command

Note: The `clightningrpc-conf` from git is required to use the `rm_subconf()` method.

Signed-off-by: Tarek <tareknaser360@gmail.com>

Signed-off-by: Tarek <tareknaser360@gmail.com>