Add scikit-learn to the example gallery

[Charlie-XIAO]

Thanks for the amazing theme! Scikit-learn has migrated to pydata-sphinx-theme for the main website since version 1.5 (scikit-learn/scikit-learn#29038), and the nice three-column layout has unlocked many potential improvements to our website UI/UX-wise. This PRs adds scikit-learn to the list of Other projects using this theme.

However though scikit-learn is not one of the earliest adopters of pydata-sphinx-theme, I'm asking if it can fit into the list of Featured projects as we have many quite some customizations that could be helpful for other users of the theme. Some of them include:

• Landing page
• API references (a searchable table on the index page containing all APIs while keeping the hierarchy in the primary sidebar)
• Each API page (customizing the secondary sidebar)
• Gallery examples (secondary sidebar customization with sphinx-gallery)
• Platform-specific installation instructions (CSS customizations based on tabs in sphinx-design)
• Dropdowns (anchor link for dropdown blocks; toggle-all button for Ctrl-F searching)
• Algolia search (WIP): DOC use Algolia for the search bar scikit-learn/scikit-learn#29666
• Many other CSS customizations

Looking forward to your feedback :)

BTW some of the projects were not placed in alphabetical order as required so I reordered a bit.

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