docs: add an Images guide covering img, svg and HTTP caching

[huacnlee]

Description

Add an Images guide under docs/ (website/docs/image.md and website/zh-CN/docs/image.md), and shorten the Image component page to the patterns applications use most, linking to the guide for details.

The guide covers:

• Sources: how each ImageSource conversion loads its bytes (URL through the App's HttpClient, asset key, file path, Arc<Image>, Arc<RenderImage>, custom loader), and that native apps must install an HttpClient before URL images load.
• Loading, decoding, and failures: the 200 ms loading delay, why with_loading and animated images need an .id(...), what triggers the fallback, format detection from bytes, and SVG rasterization in img().
• Size and fit: how auto dimensions and aspect_ratio come from the decoded image, and ObjectFit.
• svg(): path, external_path, data; why it needs its own text color and an explicit size; paint-only transformations.
• img() or svg(): a comparison table.
• Caches for decoded images: the App asset cache, scoped caches with image_cache(retain_all(...)), and a custom ImageCache example that keeps the most recently drawn images.
• Cache remote images over HTTP: wrap the App's HttpClient in a client that follows HTTP caching rules (Cache-Control, ETag/Last-Modified revalidation, shared-cache restrictions, redirect policy in the cache key, size bounds, authorization before the request), with an example that adapts http-cache-reqwest middleware (disk store) to GPUI's HttpClient. It builds one client that follows redirects and one that does not, each with its own cache, so a caller that requests RedirectPolicy::NoFollow still receives the 3xx. The approach follows a production GPUI application that installs a global caching client.
• Troubleshooting.

docs/assets.md now links to the guide. English and Chinese pages are kept in sync.

No public API changes.

How to Test

• The ImageCache and svg() examples compile against this branch's gpui-kit; the HTTP cache example was compiled and run against a local server: a repeated request is answered from the cache, NoFollow receives the 302, and a following request reaches the final 200.
• typos, astro build, and bun test tests/seo.test.ts pass; heading anchors used by cross-page links resolve.

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