docs: simplify auto-compaction documentation

[kovan]

Summary

• Replaces the version-by-version historical account (v3.0 through v3.3.3) with a clear description of current auto-compaction behavior
• Documents both periodic and revision compaction modes with dedicated subsections
• Adds recommended retention values per @ahrtr's guidance in the issue
• Documents the compaction interval rules: every hour for periods >1h, every period for periods <=1h
• Keeps the --auto-compaction-retention=10h timeline example (useful) but removes the "previously/now" framing
• Applied to v3.5, v3.6, and v3.7 docs; v3.4 left unchanged to preserve historical context

Verified against server/etcdserver/api/v3compactor/periodic.go and revision.go.

Fixes #932

[k8s-ci-robot]

Hi @kovan. Thanks for your PR.

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[jberkus]

/ok-to-test

[jberkus]

Looks good, just a few linter issues to fix: https://prow.k8s.io/view/gs/kubernetes-ci-logs/pr-logs/pull/etcd-io_website/1132/pull-website-lint/2041675759864516608

[ronaldngounou]

/lgtm

[ronaldngounou]

@kovan Could you please squash your commits? Thanks

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[jberkus]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jberkus, kovan, ronaldngounou

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