Add opera cake photos and/or diagrams to documentation

[dominicgs]

A picture is worth 1000 words and I often struggle to write 100. A picture or diagram of opera cake on the wiki page will help to make sure we're all on the same page when discussing the hardware.

Annotations:

• Ports
• Headers
• Address jumpers

┆Issue is synchronized with this Basecamp todo by Unito

[straithe]

Add annotated picture of Opera Cake to the top of https://hackrf.readthedocs.io/en/latest/opera_cake.html