New GStreamer Matrix chat space

Hello everyone,

As part of the ongoing effort to provide better tooling and services for the GStreamer community at wide, there is now a [Matrix](https://matrix.org/) instance for GStreamer available at https://matrix.to/#/#community:gstreamer.org . You can join using a [matrix client](https://matrix.org/ecosystem/clients/) and a matrix account (the GStreamer instance does not provide account creation).

The decision comes after discussions that took place during the gstreamer conference/hackfest and discussions here : https://discourse.gstreamer.org/t/a-replacement-for-irc/151 . But the summary is that Matrix provides a modern chat system with features people expect in this day and age, while not being a closed silo.

There are already some rooms up:
* [General Discussion](https://matrix.to/#/#gstreamer:gstreamer.org) The main discussion channel
* [GStreamer newcomers](https://matrix.to/#/#newcomers:gstreamer.org) for those new to GStreamer
* [GStreamer Releases](https://matrix.to/#/#releases:gstreamer.org) for preparing releases
* [Rust 🦀 GStreamer](https://matrix.to/#/#rust:gstreamer.org) for the crustaceous multimedians
* [Python 🐍 GStreamer](https://matrix.to/#/#python:gstreamer.org) for the pythonistas
* [Sysadmin and service support](https://matrix.to/#/#sysadmin:gstreamer.org) for discussions and questions on the services behind gstreamer.org (matrix, discourse, mail, ...)
* And more rooms will be made available on a as-needed basis

Note that a client with support for spaces and threads is recommended (but not mandatory).

In terms of the OFTC IRC channel, it will remain bridged to matrix (2/3 users in `#gstreamer` on IRC are already accessing it via Matrix), but we recommend switching over to using the above matrix rooms since:
* They provide the full matrix experience (history, threads, search, emojis, calls,...)
* They offer more rooms than the (single) IRC room
* Using the IRC bridge requires you to handle IRC commands if you wish to join/speak there (NickServ)
* The IRC bridge isn't maintained by the GStreamer community and we cannot guarantee it will always be around/up.