[lldb-dap][docs] Improve README (#109266)
* Document how to procure and configure the `lldb-dap` binary
* Improve documentation for `launch.json`:
* Show the examples before the reference. Most new users just want to
get going, and will only need the reference later on
* Deduplicate the list of "launch" and "attach" config settings
* Remove the `stopOnEntry` setting from "attach", since this is a
launch-only setting
* Document the previously undocumented settings `sourcePath`,
`commandEscapePrefix`, `custom{Frame,Thread}Format`, `runInTerminal`
* Add the settings `debuggerRoot` and `sourceMap` to the common section.
So far they were documented as launch-only settings.
* Document that the Debug Console prints variables / expressions by
default and that LLDB commands need to be escaped.
