Add XIAO nRF52840 + Wio SX1262 DIY Variant

[porkcube]

I figured eventually I'd get around to just making my own repo documenting it, but @Nestpebble suggested on Discord (in #general ~ 2/2 01:00UTC) it'd be worth opening a PR for variants/diy so... here's a PR complete with a README.md on how to wire up a XIAO nRF52840 to a XIAO Wio SX1262 including enabling use of the button, and how to tweak the variant.h for either i2c or serial use, or reuse a different pad to have both per the original user's specification.

Feel free to tell me to pound sound as there's already so many different hw variations, I'd be fine to have it live in my own repo (since I already motivated enough to clean up my local env and PR it proper).

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[Dylanliacc]

We will soon produce several products based on NRF52840+1262, including development kits and equipment, and I will also update the variants.