Restructure spark durable execution docs for all cluster managers

[amoghrajesh]

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Durable execution works on Spark standalone, Kubernetes, and YARN, but the docs only presented it under a heading scoped to standalone. The Kubernetes and YARN prerequisites sat inside sections framed around releasing the spark-submit JVM, so a reader looking for crash recovery had no reason to open them.

Restructuring the docs for easier navigation.

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[aaron-y-chen]

LGTM