Add new page: Telegram (closes #44)

[jwflory]

This PR adds a new page to document the Telegram group in these ways:

• Background
• Administrator action
• Administrator list

A screenshot of the rendered documentation is below:

[jwflory]

@jrtechs I pushed 21bc9ac2737899779f59f18a749fd2f77699cff7 to address your feedback:

I think the Teleirc documentation belongs separately since it is another service. But mentioning it here is a good idea, and this will also make it obvious we need to write new documentation for Teleirc.

What do you think?

[jwflory]

@Tjzabel It is already documented in the Runbook (see Other on-boarding tasks).

[Tjzabel]

My bad, didn't see it there!