docs: context directives

[shorgi]

Docs for @context and @fromContext directives.

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[Meschreiber]

Nice clear additions! Just some suggestions for readability.

One overall suggestion: it might be nice to include a very sparse code snippet and/or diagram in the introductory section (#using-contexts-to-share-data-along-type-hierarchies). I think that can often help readers "get it" faster.

[Meschreiber]

Whoops, meant to approve with my previous comments.