Be clear about electronics defaults

docs/parts/electronics/sangaboard.md says we use v0.3 but that's no longer true. We should make it consistent and say we use v5, with the nano/pico as workarounds, and mention other boards too.

The LED workaround mentions the sangaboard version, and the wiring.md text will need updating too.

We should:

• Clearly state that the version you want is a Sangaboard v5
• Clearly link to the source code, and to Filip's shop
• Create a permanent archive (e.g. Zenodo) of the PCB and firmware source, together with built binaries
• Update the firmware instructions to load the binary onto a Sangaboard v5
• Update the render on the wiring page to show a v5

Related MR is !363 (merged)

This merits proper discussion and documentation. I have written an OFEP and would welcome further discussion there.