[REF] account_anglo_saxon: refactor anglo saxon move lines

So that we can customise the real-time stock journal items made during sale invoice postings for certain companies who account for it slightly differently.
[ADD] doc: guide to the new API

Raw API documentation is not sufficient for people to learn the working
principles of the API, especially when they already know the old one. And some
things have changed since the opendays so the presentations don't reflect the
current state of affairs.

Closes #3483