docs(get-started): add GCC download URL to docs
docs(get-started): add GCC download URL to docs

Links to the diferent toolchains in the get-started documentation update according to the information in the README.md file.

Merges https://github.com/espressif/ESP8266_RTOS_SDK/pull/720

espressif/ESP8266_RTOS_SDK,espressif/ESP8266_RTOS_SDK,espressif/ESP8266_RTOS_SDK,espressif/ESP8266_RTOS_SDK,espressif/ESP8266_RTOS_SDK