Document that the chat features of Thunderbird doesn't work

Thunderbird chat features have been disabled in Tails for a long time. Since version 102 (June 2022), Thunderbird implements support for the popular Matrix chat protocol. Enabling Thunderbird chat features would allow Tails users to chat on Matrix.

Currently, Tails users don't have great options to chat on Matrix. The Matrix plugin for Pidgin is unmaintained and doesn't support end-to-end encryption. Installation of the most popular Matrix client - Element - on Tails is annoying as it is not available as an official Debian package (this guide in german explains how to install it on Tails with Flaptpak, and it isn't trivial).

Alternatively, if there's a reason for not enabling Thunderbird chat features, the reason could be documented here.

Same for IRC support.