Add documentation about adding new SIH airframes

[mbjd]

This took quite a bit of asking around and reverse engineering. Hopefully this will save the next person a bunch of time.

@reviewers, please read carefully for any inaccurate/misleading info. If unsure but there is someone else who knows this better, please request their review too.

I am not 100% certain specifically about:

• whether SENS_EN_* parameters are needed (or just recommended/usual practice)
• whether this is the right place to include things like EKF2_GPS_DELAY which is also not strictly necessary
• whether the naming template for SIH on FC is strictly necessary (and appropriate to document here), or just convention.

Thanks for any clarification.

[github-actions]

No flaws found

[hamishwillee]

Thanks. I'm on holiday. Will be back and a week ish and review then.