doc: Add new section about using Zephyr with PlatformIO

This adds a description of PlatformIO ecosystem, tutorials, examples

Signed-off-by: Valerii Koval <valeros08@gmail.com>

doc/guides/index.rst

@@ -32,3 +32,4 @@ User and Developer Guides
    west/index
    optimizations/index
    zephyr_cmake_package.rst
+   platformio/index.rst

doc/guides/platformio/index.rst

@@ -0,0 +1,91 @@
+.. _platformio:
+
+Using with PlatformIO
+#####################
+
+.. image:: img/platformio-logo.png
+  :target: https://platformio.org/?utm_source=docs.zephyrproject.org
+
+.. contents::
+    :local:
+
+What is PlatformIO?
+*******************
+
+`PlatformIO <https://platformio.org/?utm_source=docs.zephyrproject.org>`__
+is a cross-platform, cross-architecture, multiple framework, professional
+tool for embedded systems engineers and for software developers who write
+applications for embedded products. It is fully compatible with the Zephyr
+project. 
+
+- Open source, maximum permissive Apache 2.0 license
+- Cross-platform IDE and Unified Debugger
+- Static Code Analyzer and Remote Unit Testing
+- Multi-platform and Multi-architecture Build System
+- Firmware File Explorer and Memory Inspection.
+
+.. image:: img/platformio-ide-vscode.png
+  :target: https://platformio.org/platformio-ide?utm_source=docs.zephyrproject.org
+
+PlatformIO’s unique philosophy in the embedded market provides developers with
+a modern integrated development environment (`Cloud & Desktop IDE <https://docs.platformio.org/en/latest/integration/ide/index.html?utm_source=docs.zephyrproject.org#ide>`_)
+that works cross-platform, supports many different software development kits
+(SDKs) or `Frameworks <https://docs.platformio.org/en/latest/frameworks/index.html?utm_source=docs.zephyrproject.org#frameworks>`_,
+and includes sophisticated debugging (`PIO Unified Debugger <https://docs.platformio.org/en/latest/plus/debugging.html?utm_source=docs.zephyrproject.org>`_),
+unit testing (`PIO Unit Testing <https://docs.platformio.org/en/latest/plus/unit-testing.html?utm_source=docs.zephyrproject.org>`_),
+automated code analysis (`PIO Check <https://docs.platformio.org/en/latest/plus/pio-check.html?utm_source=docs.zephyrproject.org>`_),
+firmware inspection, and remote management (`PIO Remote <https://docs.platformio.org/en/latest/plus/pio-remote.html?utm_source=docs.zephyrproject.org>`_).
+It is architected to maximize flexibility and choice by developers, who can use
+either graphical or command line editors (`PlatformIO Core (CLI) <https://docs.platformio.org/en/latest/core/index.html?utm_source=docs.zephyrproject.org#piocore>`__),
+or both. PlatformIO is a must-have tool for professional embedded systems
+engineers who develop solutions on more than one specific platform. In addition,
+by having a decentralized architecture, PlatformIO offers both new and existing
+developers a quick integration path for developing commercial-ready products,
+and reduces the overall time-to-market.
+
+And it runs on any one of your favorite modern operating systems (macOS, MS Windows,
+Linux, FreeBSD).
+
+Installation
+************
+
+.. tip::
+   In our experience, PlatformIO IDE is the best way to start exploring the PlatformIO ecosystem.
+
+* `PlatformIO IDE <https://platformio.org/platformio-ide?utm_source=docs.zephyrproject.org>`_
+  is a new generation toolset for embedded C/C++ development
+* `PlatformIO Core (CLI) <https://docs.platformio.org/en/latest/core/index.html?utm_source=docs.zephyrproject.org>`_ 
+  is a unique, developed-from-scratch build system that removes the usual pain of
+  software integration, packaging, and library dependencies that developers encounter
+  when they move beyond the bounds of a specific SDK or example embedded application.
+  It can be used with a variety of code development environments and allows easy
+  integration with numerous cloud platforms and web services feeds.
+
+Configuration
+*************
+
+Please go through `the official PlatformIO configuration guide for Zephyr project <https://docs.platformio.org/en/latest/frameworks/zephyr.html#configuration>`_.
+
+Tutorials
+*********
+
+- `Zephyr and Nordic nRF52-DK: debugging, unit testing, project analysis 
+  <https://docs.platformio.org/en/latest/tutorials/nordicnrf52/zephyr_debugging_unit_testing_inspect.html?utm_source=docs.zephyrproject.org>`_
+- `Developing Zephyr RTOS embedded applications on PlatformIO and simulating on Antmicro Renode
+  <https://www.zephyrproject.org/developing-zephyr-rtos-embedded-applications-on-platformio-and-simulating-on-antmicro-renode/>`_
+
+Project Examples
+****************
+
+Please check `the official examples for various development platforms <https://docs.platformio.org/en/latest/frameworks/zephyr.html?utm_source=docs.zephyrproject.org#examples>`_
+
+Next Steps
+**********
+
+Here are some useful links for exploring the PlatformIO ecosystem:
+
+- Try `other platforms <https://docs.platformio.org/en/latest/frameworks/zephyr.html?utm_source=docs.zephyrproject.org#platforms>`_
+  that support Zephyr project
+- Learn more about `integrations with other IDEs/Text Editors <https://docs.platformio.org/en/latest/integration/ide/index.html?utm_source=docs.zephyrproject.org>`_
+- Get help from `awesome PlatformIO community <https://community.platformio.org/?utm_source=docs.zephyrproject.org>`_
+