guide: revisit Checkpoints guide(s)

[jorgeorpinel]

Per task in #2548

Trying to make it broader (more general) yet shorter, and cover all the usage details (extract from cmd ref) in a structured way.

• general text/story review
• should close guide: all the ways to use checkpoints #2432
• should close guide: Add checkpoints example that runs for a fixed number of epochs #2568

Go back to #2654 after this.

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• maybe guide: clarify resuming (checkpoints) #2742
• maybe guide: Add checkpoints example that runs for a fixed number of epochs #2568
• maybe guide: checkpoints for Tensorflow  #2509
• maybe guide: Add checkpoints example with downstream stages #2741

[jorgeorpinel]

Moving to an issue instead as it's not a high priority. See #2769 cc @iesahin (since this PR has some of the text you wrote)