Cleanup on README and boards_manager (#1693)

* Added instructions for installation with boards manager (stolen directly from esp8266)

* changed to production link instead of dev

* Added to main README. Made mods to images as requested.

* Added links for development package

* Moved version images to README.md

* Just a little change for cleaner look

* Cleaned up README.md and boards_manager.md to make installation easier.

docs/arduino-ide/boards_manager.md

@@ -8,8 +8,6 @@ Starting with 1.6.4, Arduino allows installation of third-party platform package
 - Enter ```https://dl.espressif.com/dl/package_esp32_index.json``` into *Additional Board Manager URLs* field. You can add multiple URLs, separating them with commas.
 - Open Boards Manager from Tools > Board menu and install *esp32* platform (and don't forget to select your ESP32 board from Tools > Board menu after installation).
 
-#### [Latest stable release ![Release Version](https://img.shields.io/github/release/espressif/arduino-esp32.svg?style=plastic) ![Release Date](https://img.shields.io/github/release-date/espressif/arduino-esp32.svg?style=plastic)](https://github.com/espressif/arduino-esp32/releases/latest/)
 Stable release link: `https://dl.espressif.com/dl/package_esp32_index.json`
 
-#### [Latest development release ![Development Version](https://img.shields.io/github/release/espressif/arduino-esp32/all.svg?style=plastic) ![Development Date](https://img.shields.io/github/release-date-pre/espressif/arduino-esp32.svg?style=plastic)](https://github.com/espressif/arduino-esp32/releases/latest/)
 Development release link: `https://dl.espressif.com/dl/package_esp32_dev_index.json`