Update installing to use CDN backed github links (espressif#7876)

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)

Co-authored-by: Me No Dev <me-no-dev@users.noreply.github.com>

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::