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.

This repo had no banner at all. Adding one for parity with the other library repos.

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
  • Added a centered README banner with responsive light and dark theme images.
  • Included accessible imagery highlighting TanStack.

[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: 1728a377-b423-4af0-85dd-d69805a82815

📥 Commits

Reviewing files that changed from the base of the PR and between e749554 and eeef555.

📒 Files selected for processing (1)

• README.md

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📝 Walkthrough

Walkthrough

Changes

README Banner

Layer / File(s)	Summary
Responsive README banner README.md	Adds a centered banner with light and dark image sources selected by color scheme. Includes accessible sizing metadata.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested reviewers: tannerlinsley

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change to use the dynamic README header endpoint.
Description check	✅ Passed	The description explains the changes, scope, implementation, and verification, although it uses different headings from the template.
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

🧪 Generate unit tests (beta)

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

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