zephyrSerial: Fix misdetection arduino_serial1

DT_NODE_EXISTS value is true even if the status is disabled,
so it caused misdetection.
Use DT_NODE_HAS_STATUS to detect the status of
the arduino_serial1 node is ok.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>

documentation: Add document about `builtin-led-gpios`

Add a description for `builtin-led-gpios`

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>

Add Documentation

- specify reqd changes CMakelist

- User needs to know that they will have to change the CMakelists of
their sample to find their board specific overlay

- describe how to make overlays

- Add guidance on how to map the Arduio Header in overlay files
Also rename Docs to docs to match lower case convention of other folders

Signed-off-by: Mike Szczys <mike@golioth.io>
Signed-off-by: Dhruva Gole <goledhruva@gmail.com>

- create arduino_libs to explain using external libs
This page explains how to use an external arduino library with your
project

Signed-off-by: Dhruva Gole <goledhruva@gmail.com>

update README: add Link to docs folder

Also add links to both golioth and my blog

Signed-off-by: Dhruva Gole <goledhruva@gmail.com>

- Update README: add links to the doc mds
Helps users to locate documentation more easily

Co-authored-by: Mike Szczys <szczys@users.noreply.github.com>

- Be explicit on how to add the source file to the build

- README: replace branch dev to main in installation steps

Signed-off-by: Dhruva Gole <goledhruva@gmail.com>