Add docs for collaborating with CodeTogether

[bpmct]

Document using CodeTogether with code-server

Document how to:

• Download extension (from their website)
• Run in code-server (needs --enable-proposed-api config/flag)
• Update extension (edit: they are on open-vsx)

[toddewilliams]

Thanks for adding these steps for using CodeTogether with code-server! By posting here I'll be monitoring this thread to see if any questions or issues arise.