Guide to using the new API

The raw APIdoc is insufficient for people to understand how the new API is supposed to be used, or how to port from the old API to the new one.

First draft/start of a guide-ish document for that, should serve as both an explanation of the new API and help for getting up-to-date between the old and the new API.

Want reviews from @rco-odoo @tde-banana-odoo @rim-odoo @nbessi @mart-e 
* is any important note missing? (the guide is not supposed to be exhaustive/complete)
* is anything probably unnecessary for a guide?
* errors?
* other comments?

I'm a bit uncomfortable about adding them to the guide, they're not even listed in the APIDoc currently: 
* `@api.v7`/`@api.v8` is to provide different APIs for a function depending whether it's called in v7 or in v8
* `@api.cr*` are to bypass/override the automatic adaptation of old-style methods (either because they're not recognised or because they match the wrong signature)

So in theory `@api.cr` shouldn't be needed for conversions or clean-slate development, and I've some doubts about people's ability to correctly use v7/v8