Chat guide: shorten the Matrix onboarding page and refresh its screenshots

[larnsce]

The guide under pages/get-started/chat/ is long and its ten Element screenshots date from September 2023, the last time the page changed. Raised on the landing page decision call (#142), to be done when there is time.

• Cut the page down to the steps a newcomer needs to join the openwashdata room
• Retake the screenshots with the current Element interface
• Check that the landing page Join card and the Get Involved section of the newsletter still point here

Most people who join the chat arrive through the courses, so the course material should link to the same page.