Documentation updates

[koperagen]

Let's try to put getting started snippets right in the README. One important selling point of the DataFrame is ease of working with data snippet you have: read API is straightforward and tooling helps discovering data with completion + types. Thus more emphasize on extension properties API. Some other changes are focused on making things more explicit: why we show this and that

[zaleslaw]

@koperagen could you please in your PR update the df version ctrl-c/ctrl-v to 0.12.1 in the docs?

[Jolanrensen]

@koperagen could you please in your PR update the df version ctrl-c/ctrl-v to 0.12.1 in the docs?

I'd use [VERSION] actually, otherwise we might forget it later

[koperagen]

I agree that "extension properties API" is not the right term. Later, i'll rename everything to "data schema api". Now i'd like to keep "extension properties API" to emphasize that it's some special term in our library. Consider "// execute assemble to generate extension properties". What extension properties? Not clear

[koperagen]

I updated to 0.12.1. I'd like to keep version number for peoples convenience