feat: experimental CSP docs

[ematipico]

Description (required)

This PR adds a new page under the experimental section that documents the upcoming CSP support.

For Astro version: 5.9.0. See astro PR #13802.

I leave here the link to RFC too: https://github.com/withastro/roadmap/blob/feat/rfc-csp/proposals/0055-csp.md

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Tracked Files

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en/reference/experimental-flags/csp.mdx	Localization added, will be marked as complete.

Warnings reference

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[zaitovalisher]

That's huge, cool and elegant solution that covers 90% of my needs. Great job @ematipico!

I will use your solution for google analytics, comments and styles. And Zaraz for the rest (which is ad placement, cause those are added on top of a built website and need nonces).