Fix navbar header overlap and improve docs navigation

[igerber]

Summary

• Fix title/navigation overlap: use logo.text = "diff-diff" for short navbar brand instead of the 67-char html_title
• Remove "Edit on GitHub" button (use_edit_page_button and html_context GitHub edit config)
• Reorder nav tabs: API first (most-used), then Guide, then Tutorials
• Shorten toctree captions: "API" and "Guide" instead of "API Reference" and "User Guide"
• Shorten wordy page titles in sidebar: "R Comparison", "Python Comparison", "Benchmarks"
• Promote API Reference to top of Quick Links on landing page

Methodology references (required if estimator / math changes)

• Method name(s): N/A - no methodology changes
• Paper / source link(s): N/A
• Any intentional deviations from the source (and why): None

Validation

• Tests added/updated: No test changes (docs-only)
• Sphinx build verified locally: build succeeded

Security / privacy

• Confirm no secrets/PII in this PR: Yes

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Generated with Claude Code

[github-actions]

Overall Assessment

✅ Looks good

No unmitigated P0/P1 findings.

Executive Summary

• Affected methods: none. I checked the diff against REGISTRY.md; this PR only changes docs titles, landing-page navigation, and Sphinx theme config.
• The changed surface area is limited to docs/conf.py, docs/index.rst, docs/benchmarks.rst, docs/python_comparison.rst, and docs/r_comparison.rst.
• I found no repo-local dependency on the removed edit-page GitHub context from docs/conf.py.
• I found no repo-local internal refs/tests that depend on the old page titles or their autogenerated anchors.
• The PyData theme behavior used here is consistent with upstream docs/source: search placement is controlled separately via navbar_persistent, and the navbar brand renders logo.text when present even without a logo image. (pydata-sphinx-theme.readthedocs.io)

Methodology

• Severity: none. Impact: no estimator, weighting, variance/SE, identification assumption, or default behavior changed; no methodology defect is in scope for this PR. Concrete fix: none.

Code Quality

• Severity: none. Impact: the config/title/toctree edits are internally consistent, and I found no repo-local references to the removed use_edit_page_button / html_context keys. Concrete fix: none.

Performance

• Severity: none. Impact: docs-only change; no library runtime or algorithmic hot path changed. Concrete fix: none.

Maintainability

• Severity: none. Impact: navigation labels and landing-page ordering are simpler without introducing repo-local broken references. Concrete fix: none.

Tech Debt

• Severity: none. Impact: this PR does not appear to add new deferred work, and TODO.md contains no relevant blocker for these changes. Concrete fix: none.

Security

• Severity: none. Impact: no secrets, auth/config exposure, or new execution surface is introduced by this diff. Concrete fix: none.

Documentation/Tests

• Severity: none. Impact: I found no repo-local snippet-test coupling to these heading changes. Concrete fix: none.
• Note: I could not independently reproduce a Sphinx build in this environment because sphinx and pydata_sphinx_theme are not installed here.