Document the esphome web server v2 OTA update button

[philpownall]

This is a documentation issue for users like me that use esphome devices off-internet and away from the machine that is used to compile and link the yaml. I had some difficulty figuring out:

• a) where the build files for a device are located on the computer that you are using for compiling/linking, and
• b) which file and file type the OTA update button on the web server v2 page requires to perform the update.

For reference, the firmware.bin file is the one that is required (not firmware.elf, firmware.map, or firmware-factory.bin) and it is stored in /config/.esphome/build//.pioenvs/ Now, perhaps this location is a result of using a manual esphome install on the machine that is used to compile, if so please let me know. It is quite hard to find the .bin . If there is documentation on the above please point me to it.

FWIW, I really appreciate being able to update the device from the .local web page, as there are a lot of kilometers between the compiling machine and the esp device.