Concept docs: SDK API

[gefjon]

Link to codegen page from #3847 about how you need to do that. How to connect, with link to auth page when you supply a token. Big ol' warning about advancing the connection in languages where you have to do that. Link to reducers page section about invoking reducers and registering reducer callbacks. Link to procedures page section about invoking procedures and observing return values. Link to subscription page. Docs on what connection lifecycle things we support, which is mostly the ability to disconnect and then some inconsistently implemented callbacks.