docs(docker): promote install script to Option 1, reorder install paths

[brunobuddy]

Summary

Self-hosters skimming docker/DOCKER_README.md hit the first install option they see, so burying the one-command install script at Option 3 was leaving the easiest path on the table. This PR reorders the Installation section to lead with the script and demote the manual paths.

Before

1. Option 1: Docker Compose (recommended) — 4-step manual flow
2. Option 2: Docker Run (bring your own PostgreSQL)
3. Option 3: One-command install script

After

1. Option 1: Quickstart install script (recommended) — leads with the one-shot bash <(curl -sSL ...) command; follows with the review-first 3-command flow for audit-minded users
2. Option 2: Docker Compose (manual) — same content, relabeled "manual" so the contrast is explicit. Now includes an explicit "generate a secret and paste it over the placeholder" step that mirrors what the install script does for you
3. Option 3: Docker Run (bring your own PostgreSQL) — unchanged

Also adds a one-sentence intro under the Installation heading noting that all three paths converge on the setup wizard, and updates the table of contents anchors to match the new headings.

Follows the pattern used by NocoDB, Supabase, and similar self-hosted tools — the one-liner is the first thing users see, with transparent alternatives listed below.

Test plan

• Pure docs change, no code touched
• ToC anchors validated against new ### headings (Option 1/2/3 slugs regenerated)
• npm test --workspace=packages/backend — 181 suites / 3507 tests pass
• npm run test:e2e --workspace=packages/backend — 15 suites / 107 tests pass (fresh Postgres DB)
• npm test --workspace=packages/frontend — 105 files / 2103 tests pass
• npx tsc --noEmit clean in both backend and frontend
• Manual: render on GitHub preview to confirm the new ToC links jump to the right sections

---

Summary by cubic

Promotes the one-command install script to Option 1 (recommended) in the Docker install docs so the fastest path is the first thing users see. Relabels Docker Compose as “manual” Option 2 with an explicit secret-generation step, keeps Docker Run as Option 3, adds a brief intro noting all paths end at the setup wizard, and updates ToC anchors.

^{Written for commit 9e62c6b. Summary will update on new commits.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.93%. Comparing base (0454d8c) to head (9e62c6b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1547   +/-   ##
=======================================
  Coverage   97.93%   97.93%           
=======================================
  Files         118      118           
  Lines        8581     8581           
  Branches     3230     3230           
=======================================
  Hits         8404     8404           
  Misses        175      175           
  Partials        2        2           

Flag	Coverage Δ
frontend	97.93% <ø> (ø)
shared	100.00% <ø> (ø)

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[cubic-dev-ai]

No issues found across 1 file