docs: demo page and viewer template

[cpvalente]

Until v3, we have maintained a separate repo viewer-template to teach users how to get the data from the ontime websocket server.

Meanwhile we have created a demo page, which is shipped with Ontime and already has some basic code for managing the connection
https://github.com/cpvalente/ontime/blob/master/apps/server/src/external/demo/app.js

The documentation has lagged behind and we need to update

• create documentation for demo page
• remove links to the old viewer-template
• update event-data page (currently missing auxiliary timer data)

Once users find about the demo page we can investigate whether the solution is clear enough or we should reconsider
Perhaps some users would like to advise on this

[cpvalente]

Closing this as it turns out that the auxiliary timer is already documented in runtime data.
The other tasks are accounted for and closed with #51