Update a05-board-generator.rst (#5093)
Update a05-board-generator.rst (#5093)

Added additional information about where to find the core root directory (often deeply hidden) so users can run board generator
Lan-Hekary/Arduino,sticilface/Arduino,wemos/Arduino,Lan-Hekary/Arduino,sticilface/Arduino,hallard/Arduino,sticilface/Arduino,esp8266/Arduino,Lan-Hekary/Arduino,esp8266/Arduino,wemos/Arduino,hallard/Arduino,esp8266/Arduino,Lan-Hekary/Arduino,Lan-Hekary/Arduino,esp8266/Arduino,wemos/Arduino,wemos/Arduino,sticilface/Arduino,esp8266/Arduino,sticilface/Arduino,hallard/Arduino,hallard/Arduino,hallard/Arduino,wemos/Arduino