docs: Cynthion hardware documentation

[antoinevg]

We need some basic documentation for the Cynthion hardware.

At minimum we need to have pages that cover the following topics:

• Introduction
• Minimum Host System Requirements for Cynthion
• List of Hardware Revisions
• Hardware Components
• Purchasing Information

(Linked to equivalent HackRF docs as an example)

[antoinevg]

It might also be cool to have a section with explicit rules/recommendations for people who want to build their own hardware. (e.g. something like they have in the Glasgow docs: https://glasgow-embedded.org/latest/build.html)

[martinling]

Something that would really help is a block diagram similar to the one we have for HackRF.

[mossmann]

Some information about power switching is needed. For starters, the introduction should be updated to say that CONTROL is the primary port for both control and power.

[mossmann]

The r1.4.0 hardware design release has been published: https://github.com/greatscottgadgets/cynthion-hardware/releases/tag/r1.4.0