docs: Add a CSP page #17744

[raamas]

This adds documentation page about Content Security Policy under Misc. Feedback is appreciated.

[changeset-bot]

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Latest commit: c07f0a0

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17840-svelte.vercel.app/

[teemingc]

We will probably need to make a change to the blog post link on svelte.dev to avoid this build error too:

Error: The following pages contain links to /docs/svelte/hydratable#CSP, but no element with id="CSP" exists on /docs/svelte/hydratable - see the `handleMissingId` option in https://svelte.dev/docs/kit/configuration#prerender for more info:
  - /blog/whats-new-in-svelte-january-2026