[doc] Add shape on creating a representation from another representation

[gdaniel]

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[pcdavid]

It's a more limited use case that what is described here, but note that the Representations view (in the right panel) supports displaying the structure/contents of a Portal representation (see #2897).

[sbegaudeau]

I'll rebase it and merge it but the content of the document is a bit odd.

On one side, shapes are made to describe the behavior we want to have and ADR are made to describe how we believe we implement do it. Here you have a shape talking almost exclusively about the technical details behind the feature you want to implement. And contrary to most ADR, it's a bit more a work in progress of how you found out you could to it.

That's not really an issue but in the end it feels a bit more like a "How to" document (see /doc/how-to) explaining how to create a representation using another representation as an input than either a shape or an ADR. It's still very interesting to have in the repository but we may move it / copy it there once it's fully done.

[sbegaudeau]

sate -> state

[sbegaudeau]

I've fixed it