chore(📚): keep the Canvas reference reader facing

[wcandillon]

api/canvas.mdx repeated the Getting Started → Canvas guide almost word for word (presenting frames, pixel density, transparency), and its Android section described the backing views' internals.

• Deduplication: the reference keeps one short paragraph per topic (the contract) and links to the guide for the walkthrough. Removes the link to a "resize-aware render loop" that doesn't exist, and the "Learn WebGPU for a step-by-step walkthrough" link (that page is a list of external links).
• Android rendering options: keep the two option tables, the per-view trade-offs, the remount-with-key rule, the example combinations and the translucent SurfaceView / zOrderOnTop caveat. Remove PixelFormat.OPAQUE / setOpaque, the SurfaceTexture / AHardwareBuffer routing, the compositor-signal explanation, the refresh-rate vote, the "unmeasured" latency notes and the roadmap sentence. HardwareBufferView's two caveats are kept as symptoms (shows nothing if buffers can't be allocated, possible stale or torn frames).
• The guide's intro names no backing view classes and links to the options.

Heads-up: #561, #562 and #565 also edit the Android section of this page, so whichever lands second will need a rebase. On the reader-facing side, #562's "opaque canvases use a TextureView before Android 12L" and #565's "a canvas keeps showing its last frame" are worth keeping as one sentence each; the mechanics around them (hole opening, readback, Android 11 TextureView frame pacing) are the kind of detail this PR removes.

[github-actions]

🎉 This PR is included in version 0.17.0 🎉

The release is available on:

• v0.17.0
• GitHub release

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