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 and the docs/changeset PR #13870

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).

[louisescher]

Not a direct change request, but the client router being incompatible (as discussed in the API bash) isn't mentioned anywhere yet, so that'd be nice to include.

[ArmandPhilippot]

I left some nits regarding some types, otherwise LGTM (including Sarah's suggestions). Great job Ema!

[ArmandPhilippot]

I didn't pay attention to that earlier, but I believe there is a typo each time we use style-src. According to Syntax on MDN or Content Security Policy (CSP) Quick Reference Guide, style-src is not different from the other directives: no colon, only a space.

Otherwise, LGTM!

[JusticeMatthew]

Few small things my ocd flagged flagged for me 😄