doc: move application development section

Move application development docs into the new `developing with zephyr`
section.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

doc/develop/index.rst

@@ -8,5 +8,6 @@ Developing with Zephyr
 
    getting_started/index.rst
    beyond-GSG.rst
+   application/index.rst
    modules.rst
    west/index

doc/index-tex.rst

@@ -21,7 +21,6 @@ Zephyr Project Documentation
    connectivity/index.rst
    services/index.rst
    development_process/index.rst
-   application/index.rst
    reference/index.rst
    guides/index.rst
    security/index.rst

doc/index.rst

@@ -105,7 +105,6 @@ Sections
    develop/index.rst
    contribute/index.rst
    development_process/index.rst
-   application/index.rst
    build/index.rst
    kernel/index.rst
    connectivity/index.rst