Clean-up demo scripts & documentation (#12143)

* Rewrites the demo documentation to be clearer, accurate, and moves it to our documentation tree.
* Improvements to the demo scripts:
	* `clean.sh` now runs `stop.sh` first to avoid zombie processes.
	* Uses more modern Synapse configuration (and removes some obsolete configuration).
	* Consistently use the HTTP ports for server name, etc.
	* Remove the `demo/etc` directory and place everything into the `demo/808x` directories.