Show preview.png thumbnails in the examples gallery

[FenjuFu]

Show preview.png thumbnails in the examples gallery 🖼️

The examples gallery currently renders text-only
cards. The example spec already reserves an optional preview.png per example — let's render it so
cards get a screenshot of the workflow canvas or its output.

Scope

1. Gallery component — update
   docs/.vitepress/theme/ExamplesGallery.vue
   to show a thumbnail when the example has a preview.png, with a graceful text-only fallback when it doesn't.
2. Data loader — update
   docs/.vitepress/examples.data.ts
   to expose whether each example has a preview.png and a URL the page can load (e.g. the GitHub raw URL …/raw/main/examples/<id>/preview.png).
3. Seed screenshots — add a preview.png to the three existing seed examples
   (history-knowledge-qa, ai-radio-podcast, english-essay-corrector).

Verify

cd docs && DOCS_BASE=/astron-agent/ npm run docs:build

Confirm cards with a preview.png show the image and cards without one still render cleanly.

Good for someone comfortable with a bit of Vue + VitePress. Ask here if you'd like pointers.

[mariozenmedina]

Hi! I’d be happy to help with this.

[FenjuFu]

Thanks for volunteering, @mariozenmedina, and apologies for the delayed follow-up. This task has since been implemented and merged in PR #1594, including thumbnail rendering with a text-only fallback and seed preview images, so no duplicate implementation is needed now.

If you would still like to help, testing the merged examples gallery or picking another examples task such as #1410 would be very welcome.