Add Mayfly Example sketches to Episodes 1, 2, 6

[aufdenkampe]

Given the work by @SRGDamia1 and @s-hicks2 to update the documentation and code in the EnviroDIY_Mayfly_Logger repository, it's time to start leveraging all that good material for this tutorial!

To Episode 1 :

• Into Lesson 0, link to Mayfly hardware sections of the main ReadMe, https://github.com/EnviroDIY/EnviroDIY_Mayfly_Logger
• Move "Get to know the Mayfly" section on clock battery & SD from Episode 5 to Episode 1
• Add Example sketches in this order:
  • blink_demo_1
  • blink_demo_2

To Episode 2 :

• initial_demo_blink_temperature, requires understanding of libraries

To Episode 6

• battery_measurement
• simple_file_example, which logs battery voltage to the SD card