Update/remove support room links

[AndrewFerr]

Documentation links to the Matrix support room for the upstream project (#matrix-bot-sdk:t2bot.io). We should set up a new room for this fork & link to that instead.

[AndrewFerr]

Since having our own support room might spread our support capabilities thin, it's probably better to direct people to file issues instead.

This will still involve updating links in documentation.