docs(welcome): add welcome.smd and open on container start

[nokome]

Codespace needs a welcome.smd that is automatically opended and which provides some instructions to the user on:

• how to signin/up to Stencila Cloud from within the Codespace, or if that is not possible (initial tests suggest there are issues with the callback URL flow)...

• how to signin/up to Stencia Cloud from in their browser, get a STENCILA_API_TOKEN and set it within the Codespace

• using Ctrl+F1, or the Stencila status bar item in the lower right corner, to get the Stencila Command Menu

• exploring walkthroughs from the command menu

• etc (e.g. an executable code block right there in the welcome.md that the user can just run strait away)

@jduckles, @vijay-prema will work on this today, primary around the signin flow. Could you please review his PR and add/edit where necessary.