Repository cleanup for GitHub-Mirroring

• removed docs/html and docs/documentation.pdf (are generated automatically on GitHub instance - here they would get increasingly outdated)
• changed version numbers to 2.0.0 (interface changed since Simopticon now only works with simopticon-plexe and simopticon-sumo)
• added CITATION.cff (paper now published)
• updated README to explicitly link the Simopticon website for information on the Plexe installation