Document Matrix room (and note IRC bridging).

It was noticed that, ~70% of users were accessing the project's IRC channel via the Matrix.org "portal" room.

Matrix provides some additional useful features (e.g. read receipts, message reactions, more flexible moderation, and Rust syntax highlighting in the reference Matrix client), so the project has registered #zbus:matrix.org to give the project full control of their Matrix room (to ease future room management etc.).

Link to the new Matrix room in the docs, using the matrix.to canonical URL, to permit low-friction access to the Matrix room. This makes the more featureful, and beginner-friendly "Matrix direct" option the default (retaining a reference to the bridged Freenode IRC channel for those that prefer it).