docs: add Mermaid onboarding diagrams

[leno23]

Summary

• Replaces the README architecture ASCII flow with a Mermaid flowchart.
• Adds a networked-mode sequence diagram to docs/quickstart.md for the dora up → dora start → node messaging → dora stop path.
• Replaces the top architecture stack ASCII block in docs/architecture.md with a Mermaid layered diagram.

Partially addresses the visual-diagrams portion of #1878. The debugging error table is already being handled separately in #1884.

Validation

• typos README.md docs/quickstart.md docs/architecture.md
• cargo fmt --all -- --check

Risk

Docs-only change. No runtime behavior changes.

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[heyong4725]

Reviewed. Mermaid diagrams render against GitHub's native Markdown (the mdBook docs site at guide/ is untouched, so no rendering-pipeline concerns), and the architecture-stack layer claims all match the actual workspace structure in CLAUDE.md. The sequence flow accurately captures dora up → dora start → messaging → dora stop at the right level of detail for a quickstart.

Two minor polish suggestions, neither blocking:

1. Mermaid labels with parens

README.md flowchart has Daemons[Daemon(s)<br/>per machine]. Renders fine in modern GitHub Mermaid, but the idiomatic safe form is to quote labels that contain (, ), ", [, or ]:

Daemons["Daemon(s)<br/>per machine"]

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Quoted form is future-proof against Mermaid version drift and older renderers. Optional.

2. Quickstart sequence ends one step short

The code block above the new sequence diagram in docs/quickstart.md ends with:

dora stop --all
dora down

The diagram covers up to dora stop --all (last arrow is Coordinator->>Daemon: stop nodes) but doesn't include dora down, which tears down coordinator + daemon. Adding two more arrows would complete the lifecycle:

User->>CLI: dora down
CLI->>Daemon: shutdown
CLI->>Coordinator: shutdown

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Optional but symmetric with the surrounding text.

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Thanks for picking this up. The ASCII flow → Mermaid swap is a clear onboarding upgrade.