Add data-model diagram

Closes #24

See merge request biosignal/software/mednet!17

Update documentation

See merge request biosignal/software/mednet!15

Autodoc cannot solve type aliases.
See https://github.com/sphinx-doc/sphinx/issues/10785

Update the installation and usage documentation to reflect changes made
in recent merges. Instructions relating to removed functionality has
been removed. Instructions relating to RS datasets has been commented
out for now as it needs more work.