docs/EEPROM: Add name field for the entire backpack

References #1
tools/eeprom: Rename wifi upgrade pin to pgm

This is shorter and better covers the point of the pin.
docs/EEPROM: Assume MSB first, SPI mode 0

This saves a byte per SPI descriptor, which makes the wifi backpack
descriptors fit in 64 bytes. Future backpacks reallly should have a bit
more EEPROM...

References #1
tools/eeprom: In the output, include a header per descriptor
docs/EEPROM: Require a group for every descriptor

References #1
docs/EEPROM: Mark remaining questions as future expansions

For the first version, we can leave things as they are, but the
discussions in the document might be useful for future versions.