Community page: modernise the Matrix links

Update Matrix room links to use the current standard 'matrix.to' form, as this leads to a better user experience (user gets to choose their preferred client).

Comment out the "Join our matrix rooms" button, because it linked to a matrix "group" which is now an obsolete concept replaced by matrix "spaces". (If we make a "space" then we could re-instate this button as "Join our matrix space" and update the link accordingly.)