doc: move env var section to development section

Content about environment variables belongs under introductionary
section.

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

Author: nashif

doc/guides/env_vars.rst renamed to doc/develop/env_vars.rst

@@ -8,6 +8,7 @@ Developing with Zephyr
 
    getting_started/index.rst
    beyond-GSG.rst
+   env_vars.rst
    application/index.rst
    modules.rst
    west/index

doc/develop/index.rst

@@ -10,7 +10,6 @@ User and Developer Guides
    coccinelle.rst
    code-relocation.rst
    flash_debug/index
-   env_vars.rst
    pinctrl/index
    emulator/index.rst
    platformio/index.rst