doc: board/qualcomm: document generic targets

Replace the board specific docs with a generic board.rst file which
documents the build/boot process for the sdm845 and qcs404 boards now
that the only differences are the DTB in use.

At the same time, create a debugging page to document some useful
snippets and tips for working with Qualcomm platforms.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
diff --git a/doc/board/qualcomm/index.rst b/doc/board/qualcomm/index.rst
index 0f9c429..4955274 100644
--- a/doc/board/qualcomm/index.rst
+++ b/doc/board/qualcomm/index.rst
@@ -7,5 +7,5 @@
    :maxdepth: 2
 
    dragonboard410c
-   sdm845
-   qcs404
+   board
+   debugging