Update documentation for command line usage of siibra in TVB

The current notebooks under tvb_documentation/demos/siibra do not use the latest API available in the TVB codebase for interacting with siibra. The siibra_tvb_api.ipynb notebook should be updated, removing the section regarding the API creation and leaving just the API Usage section. For this section I propose showcasing 3 APIs:

1. API for extracting just Structural Connectivities -> get_tvb_connectivities_from_kg()
2. API for extracting just Functional Connectivities -> get_connectivity_measures_from_kg()
3. API for extracting both types of Connectivities with a single call -> get_connectivities_from_kg()

The rest of the siibra notebooks showcase just how to extract different elements from the KG using siibra, so they don't really need any update.