doc: Add environemt variable overriding to get started
doc: Add environemt variable overriding to get started

This comit adds details about the overriding of environment variables such as
ESPPORT and ESPBAUD to the get started guide.

armada-ai/esp-idf,mashaoze/esp-idf,espressif/esp-idf,www220/esp-idf,www220/esp-idf,www220/esp-idf,espressif/esp-idf,armada-ai/esp-idf,www220/esp-idf,espressif/esp-idf,espressif/esp-idf,mashaoze/esp-idf,armada-ai/esp-idf,armada-ai/esp-idf,mashaoze/esp-idf,mashaoze/esp-idf,www220/esp-idf,mashaoze/esp-idf