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 root banner was the only one in this repo; the 12 public package READMEs had none. The site currently lists only react as a Charts framework, so the other adapters use ?title= rather than ?framework= - the endpoint rejects an unlisted framework instead of silently falling back. If Charts gains those frameworks on the site, these can switch to ?framework=.

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 responsive README banners across chart packages.
  • Banners automatically display optimized light- or dark-theme artwork.
  • Improved image accessibility with descriptive fallback text.
  • Updated the main README header and removed the previous local image asset.

[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: 6a36beab-43eb-40e9-a485-77a60508d94b

📥 Commits

Reviewing files that changed from the base of the PR and between 583129a and 3193f72.

⛔ Files ignored due to path filters (1)

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

📒 Files selected for processing (13)

• README.md
• packages/alpine-charts/README.md
• packages/angular-charts/README.md
• packages/charts-core/README.md
• packages/charts-scales/README.md
• packages/lit-charts/README.md
• packages/octane-charts/README.md
• packages/preact-charts/README.md
• packages/react-charts/README.md
• packages/react-native-charts/README.md
• packages/solid-charts/README.md
• packages/svelte-charts/README.md
• packages/vue-charts/README.md

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📝 Walkthrough

Walkthrough

Changes

README banner updates

Layer / File(s)	Summary
Repository README banner README.md	The local header image was replaced with a hosted responsive <picture> element that selects dark or light artwork and includes accessible metadata.
Package README banners packages/*/README.md	Package READMEs now include centered responsive banners with theme-specific image sources and descriptive image metadata.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Suggested reviewers: tannerlinsley

🚥 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 summarizes the migration of README headers to the 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

🧪 Generate unit tests (beta)

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

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