docs: rewrite Quick Start as short developer guide (CTORNDGAIN-1301)

[ymakaruk-pixel]

• Plugin-first install; MCP demoted to labeled fallback
• Bootstrap rule promoted to mandatory step for MCP mode
• Real example regenerated as in coding-flow.md
• New Customize page with examples - needs review and testing

[github-actions]

Rosetta Triage Review

Summary: This PR rewrites the Quick Start page as a developer-focused Overview with a plugin-first install path, demotes MCP to a fallback, and adds a new Customize page (209 lines) covering context files, project rules, and helper MCPs — all as website documentation changes only (no code logic modified).

Findings:

• Scope: Focused and coherent — 4 files changed, all in docs/web/. Adds a new Customize guide and substantially rewrites the Quick Start page.
• Description: Adequately describes the 4 key changes; the author notes the Customize page "needs review and testing," which aligns with the documentation label added.
• Breaking changes: None to code or APIs. The Quick Start page H1 was renamed from "Quick Start" to "Overview" — users with bookmarked anchor links to that heading may be affected.
• Documentation: This PR is the documentation; no public API or code changes require additional docs.
• Test coverage: No tests applicable — Jekyll docs site only.
• Internal links: Several links in customize.md reference /rosetta/docs/architecture/ and /rosetta/docs/quickstart/ — the latter is unchanged in permalink (/docs/quickstart/) so it resolves correctly. Worth a local Jekyll build to confirm all relative links render properly.

Suggestions:

• Run a local Jekyll build (bundle exec jekyll serve) to verify the new Customize page renders, the sidebar Customize link appears in the correct position, and all internal cross-links resolve.
• Consider whether renaming the Quick Start H1 to "Overview" matches user expectations — returning users navigating by heading may find this confusing; the permalink /docs/quickstart/ is unchanged which is good.
• The PR body mentions "Real example regenerated as in coding-flow.md" — confirm the example in the new quickstart.md accurately reflects the current coding workflow steps.

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