feat(FR-2711): add docs site production uplift feature spec (#6988)

Resolves FR-2711

## Summary
- Add feature spec for production-level improvements to the webui-docs static site (Option A — incremental in-house toolkit improvements, no SSG migration)
- 6 feature buckets / 6 sub-tasks under epic FR-2710:
  - **F1** — Site entry & multilingual routing
  - **F2** — SEO & sharing metadata
  - **F3** — Information architecture (sidebar grouping + right-rail TOC + breadcrumbs)
  - **F4** — Reading UX: code blocks (Shiki syntax highlighting + Copy button — dark mode deferred to a separate follow-up)
  - **F5** — Build robustness & static asset optimization
  - **F6** — Versioned docs & version selector (minor-grained; eligibility limited to minors that build cleanly on the current toolkit; past-minor artifacts stored on GitHub orphan branches `docs-archive/<version>`)
- Cross-cutting acceptance: every bucket's PR must verify **PDF pipeline (`build:pdf`) does not regress** — both pipelines share `markdown-processor.ts` and `book.config.yaml`
- Hosting / AWS Amplify deployment pipeline (how the orphan-branch archives get merged and exposed publicly) is **deferred to a separate deployment spec** to keep this spec focused on the toolkit
- Epic: FR-2710

## Test plan
- [ ] Spec file renders correctly on GitHub
- [ ] Each bucket has clear, testable acceptance criteria
- [ ] Cross-cutting PDF non-regression criteria are explicit and actionable
- [ ] No overlap with FR-2159 / FR-2160 (both Done)

Author: yomybaby

.specs/FR-2710-docs-site-production-uplift/metadata.json

@@ -0,0 +1,11 @@
+{
+  "epicKey": "FR-2710",
+  "epicUrl": "https://lablup.atlassian.net/browse/FR-2710",
+  "slug": "docs-site-production-uplift",
+  "specTaskKey": "FR-2711",
+  "specTaskUrl": "https://lablup.atlassian.net/browse/FR-2711",
+  "stories": [],
+  "sourceIssues": [],
+  "predecessorEpics": ["FR-2159", "FR-2160"],
+  "createdAt": "2026-04-25T00:00:00Z"
+}