Update scikit-learn documentation (1.9.1)

[ridge-kimani]

Updates scikit-learn to 1.9.1 (released September 11, 2026). The 1.9 docs URL is unchanged, so only self.release moves.

Regenerating also picks up the MathFilter from #2752, so this is the first documentation whose formulas render as native MathML on DevDocs. Tracked in #2763 (refs #1264).

Verified locally with thor docs:generate scikit_learn --force:

• 944 pages generated without errors; index has the same 3,835 entries and 61 types as the production 1.9.0 index (two example pages were renamed upstream)
• 159 pages contain math, 2,805 <math> elements, no _math-fallback left behind and no unrendered \(...\) / \[...\] delimiters
• Checked the user guide pages (for example modules/preprocessing, modules/lda_qda, modules/svm) and API reference pages in the app, in light and dark themes

If you're updating existing documentation to its latest version, please ensure that you have:

• Updated the versions and releases in the scraper file
• Ensured the license is up-to-date (still BSD-3-Clause, COPYING in the scikit-learn repo)
• Ensured the icons and the SOURCE file in public/icons/your_scraper_name/ are up-to-date if the documentation has a custom icon (refreshed with 1.9.0)
• Ensured self.links contains up-to-date urls if self.links is defined
• Tested the changes locally to ensure:
  • The scraper still works without errors
  • The scraped documentation still looks consistent with the rest of DevDocs
  • The categorization of entries is still good

🤖 Generated with Claude Code