Grab SDK docs from entropyxyz/sdk repo.

[johnnymatthews]

The mixmix/DOCS branch in entropyxyz/sdk has a bunch of useful docs related to the SDK. Let's import those docs into /reference/sdk.

[johnnymatthews]

@mixmix mentioned something about the SDK being in a funky state, so the docs he generated might be out of date. Let check in with him before we try and bring any SDK docs through to this repo.

[mixmix]

yeah once 0.12.0 is out this will be way less chaotic

[johnnymatthews]

I'm gonna move this back to the backlog for now. It's not exactly "blocked", but it's also not something we're gonna work on in the next couple of days.