MinIO updates in ECE docs

[eedugon]

In relation with https://github.com/elastic/cloud/issues/134936, we have realized that the doc /deploy-manage/tools/snapshot-and-restore/minio-on-premise-repository.md was really outdated and needed some love.

This is the main page to review:

MinIO self-managed repository

Applied changes:

• Changed wording to MinIO across all docs. No minio or Minio anymore.
• Fixed a lot of broken links to MinIO docs.
• Removed screenshots (thanks @Kushmaro for pointing that out), and replacing them by proper links and instructions.
• Adding a note to recommend installing MinIO in separate hosts, although we support installing it in ECE allocator hosts.
• Removed the manual steps to restore a snapshot from another cluster as now that's offered and documented as an ECE feature. I've linked to the right doc for that.
• Improved the structure of certain sections as they were hard to understand (the bucket creation step for example was incorrect).
• Renamed offline to air-gapped (for consistency with the new IA and new docs).
• Changed title from on-premise to self-managed (because ECE hosts or the customer minio hosts could be on-premises or on cloud VMs technically. What they are in reality is self-managed).
• Added useful links to other parts of the documentation.
• Removed the statements about 6.x and 5.x versions of Elasticsearch. These are unneeded as in ECE 4.0 it's mandatory for all deployments to be on 8.0.0 or above. So there's no need to mention it.

Closes https://github.com/elastic/cloud/issues/134936

[github-actions]

🔍 Preview links for changed docs

• deploy-manage/deploy/cloud-enterprise/add-custom-bundles-plugins.md
• deploy-manage/tools/snapshot-and-restore/cloud-enterprise.md
• deploy-manage/tools/snapshot-and-restore/manage-snapshot-repositories.md
• deploy-manage/tools/snapshot-and-restore/minio-on-premise-repository.md

[kilfoyle]

LGTM! 🎸
Very nice!