Use stable documentation of PlatformIO (#2337)

Author: ivankravets

README.md

@@ -67,14 +67,14 @@ development with cross platform build system, library manager and full support
 for Espressif (ESP8266) development. It works on the popular host OS: Mac OS X, Windows,
 Linux 32/64, Linux ARM (like Raspberry Pi, BeagleBone, CubieBoard).
 
-- [What is PlatformIO?](http://docs.platformio.org/en/latest/what-is-platformio.html)
-- [PlatformIO IDE](http://platformio.org/#!/platformio-ide)
-- Quick Start with [PlatformIO IDE](http://docs.platformio.org/en/latest/ide/atom.html#quick-start) or [PlatformIO CLI](http://docs.platformio.org/en/latest/quickstart.html)
-- [Advanced using](http://docs.platformio.org/en/latest/platforms/espressif.html) -
+- [What is PlatformIO?](http://docs.platformio.org/en/stable/what-is-platformio.html)
+- [PlatformIO IDE](http://platformio.org/platformio-ide)
+- Quick Start with [PlatformIO IDE](http://docs.platformio.org/en/stable/ide/atom.html#quick-start) or [PlatformIO CLI](http://docs.platformio.org/en/stable/quickstart.html)
+- [Advanced using](http://docs.platformio.org/en/stable/platforms/espressif.html) -
   custom settings, uploading to SPIFFS, Over-the-Air (OTA) or using stage version
-- [Integration with other IDE](http://docs.platformio.org/en/latest/ide.html) -
+- [Integration with other IDE](http://docs.platformio.org/en/stable/ide.html) -
   Atom, CLion, Eclipse, Emacs, NetBeans, Qt Creator, Sublime Text, VIM and Visual Studio
-- [Project Examples](http://docs.platformio.org/en/latest/platforms/espressif.html#examples)
+- [Project Examples](http://docs.platformio.org/en/stable/platforms/espressif.html#examples)
 
 ### Building with make