feat: document experimental static headers

[ematipico]

Description (required)

This PR adds documentation of experimentalStaticHeaders for the Node.js adapter and Netlify adpater.

For Astro version: 5.11. See astro PR #13972.

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en/guides/integrations-guide/netlify.mdx	Source changed, localizations will be marked as outdated.
en/guides/integrations-guide/node.mdx	Source changed, localizations will be marked as outdated.
en/guides/integrations-guide/vercel.mdx	Source changed, localizations will be marked as outdated.

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[sarah11918]

OK, this probably needs correcting, but I tried to make this sound first, a bit generic, and then, use CSP as an example of when/how it's used. Lists with one item always feel weird, so maybe we can get around that! 😂

[ArmandPhilippot]

I noticed two typo, and I wonder if experimentalStaticHeaders is in the right place for the Netlify adapter.

We are not excluding/including files but setting custom CSP directives per page, right? So, we could use an h3 instead of an h4 but then, this heading could be inconsistent with others under Usage. Should we have something like the following?

## Usage
...
### Custom headers for prerendered pages
Some intro?
#### `experimentalStaticHeaders`
The current content.

[sarah11918]

Looks great, thank you @ematipico ! This will be good to merge tomorrow!

[ematipico]

@sarah11918 and @ArmandPhilippot The commit dded21a (#11971) update the Vercel adapter with the same feature

[ArmandPhilippot]

Only a missing p tag, otherwise LGTM!