[lldb][Docs] Add page about debugging lldb itself (#65332)

We have docs about how to use lldb on other programs, this tells you how
to use lldb on ldlb and lldb-server.

Lacking any Mac experience I've not included any debugserver information
apart from stating it will be similar but not the same.

I plan for this page to include sections on debugging tests and other
things but this initial commit is purely about the two main binaries
involved.