doc: move flashing/debugging section to developing with zephyr

This section belongs in the intro about developing with Zephyr.

Signed-off-by: Anas Nashif <[email protected]>

Author: nashif

doc/guides/flash_debug/host-tools.rst renamed to doc/develop/flash_debug/host-tools.rst

@@ -10,5 +10,6 @@ Developing with Zephyr
    beyond-GSG.rst
    env_vars.rst
    application/index.rst
+   flash_debug/index
    modules.rst
    west/index

doc/guides/flash_debug/index.rst renamed to doc/develop/flash_debug/index.rst

@@ -8,7 +8,6 @@ User and Developer Guides
 
    coccinelle.rst
    code-relocation.rst
-   flash_debug/index
    platformio/index.rst
    portability/index.rst
    porting/index