docs: add a from-zero overview with architecture diagrams

[ttncode]

What this changes

• Adds docs/README.md, the from-zero entry page: what scaffold is, repository map, how scaffold new works, what a generated project contains, commands table, glossary, reading path.
• Adds six diagrams under docs/diagrams/ (HTML source + exported SVG): system context, code layers, scaffold new, generated project, release flow, scaffold update. Each node was derived from the code, not from existing prose.
• README.md: links the new page first under Documentation.

How it was verified

• Every command, flag, path and function in docs/README.md and the diagrams checked against scaffold, lib/*.sh, common/ and the reusable workflows; an independent review re-checked them.
• SVGs: parsed as XML, rendered in headless Chrome at 830px (GitHub column width); smallest text ≥ 8px, no clipping.
• bats tests/documentation.bats 5/5 (a deliberately broken path and image in the new page made the path and link tests fail). mise run test-unit 199/199. mise run lint clean.

Checklist

• mise run lint passes
• mise run test-runner passes — docs only; CI runs it
• New behaviour has a test that fails without the change
• Docs that describe changed behaviour were updated in the same commit
• No unrelated changes