docs: polish - version switcher, robots.txt, notebook H1 normalization, llms.txt landing links

[igerber]

Summary

• Final polish pass of the RTD docs UX overhaul (after docs: restructure RTD site around 5 top-level sections (IA overhaul) #703 IA restructure, test: CI-enforced docs IA guards (tests/test_docs_ia.py) #706 IA guards, docs: fix search-result excerpts (CSS-special anchors) + enable search-as-you-type #713 search fix); every item was scoped independently droppable.
• Navbar version switcher: docs/_static/switcher.json (stable + latest, preferred: stable), version-switcher added to navbar_end, version_match from READTHEDOCS_VERSION. json_url points at the latest build so future entries propagate to all published versions without config changes. check_switcher stays off - the build-time URL probe would fail the -W build until the json first publishes; the dropdown populates client-side. ONE-SELECTOR POLICY (deploy coupling): the RTD flyout addon (Settings -> Addons -> Flyout menu) is being disabled in the RTD dashboard so the site has a single version control - decided after review flagged that the flyout (50 active versions) plus the navbar switcher would mean two selectors with different lists; documented in conf.py.
• docs/robots.txt served at the domain root via html_extra_path: disallows only /en/*/_modules/ (~60 thin autogenerated source-view pages), preserving the sitemap line. RTD's autogenerated default allowed everything.
• Dropped "Tutorial N:" prefixes from 8 notebook H1s (08, 18, 19, 20, 21, 22, 24, 27) so page titles and search-result snippets lead with the topic. Page URLs unchanged; only the H1's own fragment id changes, which resolves to the page top the bare URL already opens; sidebar labels were already short-form toctree overrides. Same byte-identity JSON round-trip gate as docs: fix search-result excerpts (CSS-special anchors) + enable search-as-you-type #713.
• Linked the 5 section landing pages from diff_diff/guides/llms.txt.
• Card copy refinement deliberately dropped - reviewed twice since docs: restructure RTD site around 5 top-level sections (IA overhaul) #703 and reads well.

Methodology references (required if estimator / math changes)

• Method name(s): N/A - no methodology changes (diff_diff/guides/llms.txt is documentation data; no estimator code touched)
• Paper / source link(s): N/A
• Any intentional deviations from the source (and why): None

Validation

• Tests added/updated: No test changes (existing suites cover the surfaces: docs IA guards, guides/discoverability tests, tutorial-drift suites, search smoke test)
• Backtest / simulation / notebook evidence (if applicable): -W build clean; robots.txt + switcher.json + revised titles verified in built HTML; switcher button browser-verified (the only console entry is the expected 404 on the not-yet-published switcher json); search smoke test passes; 192 guides/IA/drift tests + 172 doc-deps tests pass; all 8 edited notebooks validate against the nbformat schema.

Security / privacy

• Confirm no secrets/PII in this PR: Yes

[github-actions]

Overall assessment: ✅ Looks good

Executive summary

• Documentation-only changes; no estimator, inference, assumptions, or defaults changed.
• Switcher JSON, landing-page targets, and robots directives validate correctly.
• Notebook edits only normalize H1 titles; no repository links target the removed fragments.
• One minor test-coverage observation; no P0/P1 findings.

Methodology

No findings. No methodology registry or paper contracts are affected.

Code Quality

No findings. Configuration and JSON are well-formed.

Performance

No findings.

Maintainability

No findings.

Tech Debt

No findings.

Security

No findings or exposed secrets.

Documentation/Tests

• P3 — New documentation assets lack explicit regression guards
  • Location: docs/conf.py:L108-L149, docs/_static/switcher.json:L1-L13, docs/robots.txt:L1-L10
  • Impact: Future edits could silently break the client-side switcher or remove the robots override because check_switcher=False disables build-time validation.
  • Concrete fix: Optionally add assertions to tests/test_docs_ia.py covering the switcher schema, html_extra_path, and robots directives.

Static validation passed. The full test suite was not run because pytest is unavailable in the review environment.