Add docs for experimental incremental static builds

[matthewp]

Description (required)

Adds a new experimental reference page about the upcoming incremental builds feature.

References

For Astro version 7.2

withastro/astro#17084

[astrobot-houston]

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File	Note
en/reference/experimental-flags/incremental-build.mdx	Localization added, will be marked as complete.

Warnings reference

Icon	Description
🔄️	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.

[github-actions]

Preview deployment

✅ Deployment complete!

• Latest commit: 2e68a81
• Preview: https://docs-incremental-static-builds.previews.docs.astro.build
• Workflow run: View logs

[ArmandPhilippot]

This looks good, thanks Matthew!
But, I think we miss something about content collections digest. Someone building a custom loader might want to try this feature, and we don't explain how they can provide a digest/what is the type of digest.

[matthewp]

You're a tad early @ArmandPhilippot, still in draft because I'm not quite done. Will incorporate your suggestions 😄

[ArmandPhilippot]

Ah, sorry, I'll let you finish. 😄 I had a doubt as the other one (#14336) is also in draft.

[matthewp]

@ArmandPhilippot this is ready for review now

[ArmandPhilippot]

Thanks Matthew! I left two nits, but nothing blocking.

[ArmandPhilippot]

Thanks again! I'm updating the branch and merging this, so we have a single PR to merge for the 7.2 release.