Adds conceptual description to the Access isolation overview page

[kosabogi]

This PR adds details to the 'Access isolation for the found-snapshots repository' overview page.

Related issue: https://github.com/elastic/docs-projects/issues/447

[eedugon]

Some changes suggested:

• Remove the note and make the content part of the introduction.
• Change the initial sentence as the explained default was incorrect.

[eedugon]

We need to clarify what's the default, because this sentence is contradictory with the note, which was the only content of the original doc (https://www.elastic.co/guide/en/cloud/current/ec-snapshot-repository-migration.html).

I believe the default now is isolation. In such case the introductory sentence might cause confusion.

[eedugon]

Suggested change

	In {{ech}}, [snapshots](/deploy-manage/tools/snapshot-and-restore.md) are stored in a repository. By default, deployments in the same region may have access to each other’s snapshots through the `found-snapshots` repository.
	To enhance security, access isolation ensures that each deployment can only access its own snapshots. This prevents accidental or unauthorized access to backups from other deployments within the same organization.
	In {{ech}}, access isolation ensures that each deployment can only access its own snapshots, preventing accidental or unauthorized access to backups from other deployments within the same organization.
	Any newly created deployment has snapshot isolation set up by default. These guides are only relevant for deployments created before this default was introduced, where deployments within your organization in the same region may have access to each other's snapshots.
	If a deployment can access the snapshots of other deployments, a notification will appear in the deployments menu under Elasticsearch > Snapshots, prompting you to set up access isolation.

[eedugon]

Looks great now!