docs: add building-for-mobile guide

[OneEyed1366]

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Description(required)

Adds a "Building for mobile" guide under Guides, closing the docs gap in #1636: there was no answer to "can I build a mobile app with Solid?" and solid-js/universal (createRenderer) had no mention anywhere in the docs.

Covers three routes:

• Keeping the DOM renderer — Capacitor, Tauri (WebView), NativeScript (DOM shim over native views)
• Native views via solid-js/universal — the createRenderer API, with SolidTV and Symbiote Native as production examples

Also notes solid-native is deprecated — flagged maintained: false on solidjs.com's Resources page, per the issue's own data point.

Related issues & labels

• Closes [Request]: Cover native/mobile rendering (solid-js/universal) - the docs have no answer to "can I build a mobile app with Solid?" #1636
• Suggested label(s) (optional): 'enhancement', 'documentation'

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