docs: organize public theme guides

[rdlabo]

Summary

• streamline the public feature guide and remove markup details duplicated in the special-markup guide
• clarify the required structure for grouped Ionic items, including framework-neutral usage notes
• improve the experimental animation guide and README links for the current prerelease
• keep special markup examples together in one canonical document
• document the optional MD3 pairing, shared Sass order, dark-mode requirements, and cross-platform animations

Validation

• npm run build
• Prettier check
• git diff --check
• maintainer review
• third-party reader review

Related

The Japanese portal translations will be updated in rdlabo-dev/website#11 after this PR is merged.

[github-actions]

Playwright test results

 90 passed

Details

 90 tests across 2 suites
 1 minute, 54 seconds
 b8962bd
 This detailed result covers Ionic 9 only. Ionic 8 runs against the same screenshots in a separate matrix job; check the workflow run for both results. To update the screenshots, comment with /update-screenshots.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-ios26/pr-119/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.