ADR01 Modelator architecture

[andrey-kuprianov]

Closes: #210 on ADR01-Modelator Architecture (rendered). This is very much WIP, and is subject to many changes and revisions. Draft PR is created to facilitate early feedback.

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For contributor use:

• If applicable, unit tests are written and added to CI.
• Ran go fmt, cargo fmt and etc. (or had formatting run automatically on all files edited)
• Updated relevant documentation (docs/) and code comments.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Re-reviewed Files changed in the Github PR explorer.
• Added a changelog entry, using unclog.