docs: use the dynamic README header endpoint

[AlemTuzlak]

Adopts the dynamic README header endpoint added in TanStack/tanstack.com#1076,
which is merged and live.

README banners now point a <picture> at https://tanstack.com/api/readme/
instead of a committed PNG. The endpoint renders 1800x450 in light and dark, so
a branding change lands in every README at once and dark-mode readers get a
dark banner.

The banner was media/headerv1.png, and the package READMEs referenced it relatively, so they were already broken on npm.

Notes

• Light and dark are served through <picture>, per GitHub's guidance. The trailing <img> stays the light variant, as the fallback for renderers that ignore <picture> (npm, most editors).
• Only packages a user installs directly get a banner. Private and internal packages are deliberately left alone.

Verification

Every URL in this diff was requested against the live endpoint and returned
200 image/png at 1800x450. git grep confirms no README still references
the deleted asset.

Summary by CodeRabbit

• Documentation
  • Updated project README header images to adapt to light and dark themes.
  • Improved image accessibility with descriptive alternative text.
  • Added responsive image sizing metadata for more consistent rendering.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 58d22e1b-3284-4cb7-b6eb-cb492584e601

📥 Commits

Reviewing files that changed from the base of the PR and between fef4517 and 0ca0974.

⛔ Files ignored due to path filters (1)

• media/headerv1.png is excluded by !**/*.png

📒 Files selected for processing (3)

• README.md
• packages/ranger/README.md
• packages/react-ranger/README.md

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📝 Walkthrough

Walkthrough

The repository and package READMEs now use responsive, theme-specific header images with accessibility metadata.

Changes

README header images

Layer / File(s)	Summary
Responsive README images README.md, packages/ranger/README.md, packages/react-ranger/README.md	The static header images now use <picture> elements with dark- and light-theme sources. The images include descriptive accessibility text, and the root README sets a fixed width.

Estimated code review effort: 1 (Trivial) | ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: replacing committed README header assets with a dynamic endpoint.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch readme-header-endpoint

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