Convert architecture/core concepts tutorial into Pluto notebook

[pwochner]

• Convert from md to Pluto notebook.
• Make tutorial text more concise.
• Make sure tutorial structure and description in overview match.

See also #122

See draft PR #155 for latest version of the tutorial.

[pwochner]

While moving Architecture and Core Concepts to a Pluto notebook, I noticed that some of the sections are missing or incomplete:

• Finish program iterators
• Add section on problem specification

[ReubenJ]

@pwochner started work in https://github.com/Herb-AI/Herb.jl/tree/documentation-core-concepts