Adds some entry-level and developer-friendly docs for the debug adapter.
The one in `docs/dap.md` is meant for someone trying to use the debugger
while the one in `docs/reference/buildx_dap_build.md` is more focused on
documenting the command to be integrated in a debugger extension.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
