Implementer's guide: notes on contextual execution (#1525)

* Add notes about contextual execution

* Clarify that `validation_data_hash` consists of global and local data

* Update roadmap/implementers-guide/src/runtime/inclusion.md

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Incorporate rphmeier's suggestion.

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
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