Link to additional Matrix chat rooms from community page

[JasonWeill]

The community page links only to the Gitter room for jupyter/jupyter, which is very low in traffic:

jupyter.github.io/community.md

Lines 33 to 37 in 54527f0

	- title: Jupyter Gitter Chatroom
	description: A real-time chatroom, for general development related discussions.
	src: assets/community/gitter.svg
	alt: Gitter logo
	url: https://gitter.im/jupyter/jupyter

Neither the community page nor the Gitter jupyter/jupyter room link to other official Gitter/Element/Matrix rooms, such as jupyterlab/jupyterlab, which are more active. Should we add these links on the Jupyter community page?