docs: organize documentation (Diátaxis + C4 mermaid)

[henrypalacios]

Summary

• Replace the forked Uniswap README with an engram-style landing pointing at the new docs tree.
• Add a docs/ scaffold following Diátaxis: overview, glossary, per-package reference, flows, diagrams, ADRs.
• Add C4 mermaid diagrams (context, containers) and a sequence diagram for trade execution.
• Document the protocol-core / sdk package split and the JSBI choice as ADRs.

Structure

README.md                    # engram-style landing
docs/
├── overview.md              # what + why
├── glossary.md              # domain terms
├── packages/
│   ├── protocol-core.md
│   └── sdk.md
├── flows/
│   ├── trade-execution.md
│   └── chain-onboarding.md
├── diagrams/
│   ├── c1-context.mmd
│   ├── c2-containers.mmd
│   └── flow-trade.mmd
└── adr/
    ├── 001-protocol-core-split.md
    └── 002-jsbi-over-bigint.md

All docs respect Diátaxis line caps (reference ≤40, explanation ≤50, landing ≤80). Mermaid is raw .mmd — GitHub renders it natively in markdown previews.

Test plan

• Open the rendered README on GitHub and click each docs link
• Verify the three mermaid diagrams render in the GitHub preview
• Skim each doc — confirm tone, accuracy of public exports, no broken cross-links