[do not merge] feat: turn intro and fundamentals into a common intro section

[salmad3]

Context

• Continues with introduction section improvements and combines the fundamental section with better oriented subheadings (i.e. core abstractions and protocol basics).
• This PR is only focussed implementing planned section level changes and will be followed by targetted change requests that populate the associated documents.
• resolves #249, #32, #18

To-Do

• [ ] Re-check content
• [ ] Update links to relative path

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[salmad3]

Conflicts and link fixes will be applied in this PR once all associated material has been added.