Add documentation for block nesting

[SamyPesse]

• Document RawContentState with blocks property
• Keys should not contain /
• NestedUtils

[mitermayer]

@SamyPesse do you think you could take over the documentation bit ?

[mitermayer]

Will assign this to you @SamyPesse hope you dont mind :D

[bradleyayers]

I think it'd be useful to make the pull request description more elaborate to give reviewers (and the community) a clearer idea of the changes and consequences of such.

What I'm thinking is something along the lines of microsoft/TypeScript#7140 which provides the background and conceptual rationale in addition to concrete technical examples.