Changes

refs #2820: Add synchronous language elements to ModelicaReference (#3443)

* Add main section for synchronous and state machines.

* spelling

* Add state machine operators.

* Synchronous elements as basically place-holders.

* Remove multiple blank lines.

* Correct alphabetical order and remove trailing white spaces.

* Fix HTML errors as reported by CI tidy

* Remove empty paragraph.

* Fix typos

* Refer to HTML version of Modelica Specification

Co-authored-by: Dietmar Winkler <dietmar.winkler@dwe.no>
Co-authored-by: Thomas Beutlich <modelica@tbeu.de>