Reorder parts

[SergioGasquez]

Basically this:

• Deletes the old part 3
• Old part 4 (mqtt with env variables) becomes part 3
• Old part 5 (mqtt with provisioning) becomes part 4
• Old part 6 (mqtt with provisioning and OTA) becomes part 5

[bjoernQ]

removing part 3 makes a lot of sense to me

for similar reasoning I wonder if "part 2" makes a lot of sense (didactically wise) - I see we want to teach users how to connect to WiFi and to make it more interesting we also show how to send sensor data via http - but in the part we throw that away and replace it with MQTT and we show using HTTP again in part 🤷‍♂️

[bjoernQ]

I admit I didn't try to really review this but since it's just shuffling around the parts, LGTM