Add register library topic to design guide

[tannewt]

No description provided.

[dhalbert]

Thanks for the examples. @caternuson look over too; I myself have never written a register-based driver.

[caternuson]

Thanks for writing this and updating this usage guidance.

Worth adding verbiage that it's also specific to being over I2C? Add mention of the SPI version?

When possible, use it for unpacking and packing registers.

Is there an easy example that could be added for when it would not be possible?

[caternuson]

Thanks! Looks good. I think having the counter example really helps.