Add a sequence diagram for the attach scenario

https://microsoft.github.io/debug-adapter-protocol/overview has a really nice sequence diagram that helps a lot for the launch scenario. It would be great if a similar figure for the attach scenario could be added, right now I'm a bit lost how that is working.

In particular: the launch scenario suggests that we start the debugger in the intialize event. But at that point I don't yet know whether the DA is asked to run in attach or laucnh mode, and I need to start the debugger in a different way, depending on which of those I should do. So I guess I shouldn't really start the debugger in initialize, but somewhere else...

But ideally the debugger would be already running when breakpoint information is sent. So do I need to "lazily" start the debugger in the first breakpoint request? Or something like that?

Start with the launch sequence diagram at https://microsoft.github.io/debug-adapter-protocol/overview and trace the attach scenario described in this issue. Determine when initialization, debugger startup, and breakpoint requests occur for attach mode, then add a matching sequence diagram that makes the ordering clear.