doc: guides: add pinctrl guide

Add a user guide that provides general concepts on pin control, details
on Zephyr model, implementation guidelines, etc.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/guides/index.rst

@@ -17,6 +17,7 @@ User and Developer Guides
    debug_tools/index
    device_mgmt/index
    dts/index
+   pinctrl/index
    emulator/index.rst
 
    modules.rst