Document snapshot repositories at deployment level on ECE

[eedugon]

ECE snapshot repositories documentation only covered creating repositories at platform level, used by ECE for automatic snapshots of the deployments (found-snapshots).

ECE users could also configure repositories directly at deployment level, independently of the automatic snapshots handled by the platform.

This PR covers that use case by linking ECE users to the ECH specific docs for custom snapshot repositories, which cover the exact use case that ECE is missing in the docs.

Closes #1494

@shainaraskas , before checking with ECE PMs I'd to get your review.

I also update the applies_to tags and include a note in the 3 ECH affected documents that now can be used by ECE.

• pending TBD - We have to add a link to the new content in /manage-data/migrate.md when it says:

Although these instructions focus on Elastic Cloud Hosted, you should follow the same steps for Elastic Cloud Enterprise by configuring the repository directly at the deployment level.

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Vale Linting Results

Summary: 1 suggestion found

💡 Suggestions (1)

File	Line	Rule	Message
deploy-manage/tools/snapshot-and-restore/cloud-enterprise.md	21	Elastic.Wordiness	Consider using 'also' instead of 'In addition'.

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🔍 Preview links for changed docs

• deploy-manage/tools/snapshot-and-restore/cloud-enterprise.md
• deploy-manage/tools/snapshot-and-restore/ec-aws-custom-repository.md
• deploy-manage/tools/snapshot-and-restore/ec-azure-snapshotting.md
• deploy-manage/tools/snapshot-and-restore/ec-gcs-snapshotting.md