Update installing to use CDN backed github links

This increases the reliability of users hitting the boardsmanager urls, improving speed and avoiding request throttling, by using the github CDN frontend as mentioned in this old ticket:
espressif#3505 (comment)

Author: tyeth

docs/source/installing.rst

@@ -25,11 +25,11 @@ This is the way to install Arduino-ESP32 directly from the Arduino IDE.
 
 - Stable release link::
 
-   https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_index.json
+   https://espressif.github.io/arduino-esp32/package_esp32_index.json
 
 - Development release link::
 
-   https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_dev_index.json
+   https://espressif.github.io/arduino-esp32/package_esp32_dev_index.json
 
 
 .. note::