Add EKS AWSManagedMachinePool example to spot instances doc

[pydctw]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
Add an example how to use spot instances with a EKS managed node group backed by AWSManagedMachinePool.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2523

Special notes for your reviewer:

Checklist:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

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[sedefsavas]

This is correct, good to have a note or warning on this here if there is no other warning in the doc.

[Ankitasw]

+1

[pydctw]

A good point. Slightly modified the note from MachinePool to AWSMachinePool as one variation of machine pool implementation, AWSManagedMachinePool, supports spot instance and moved to the end.

[sedefsavas]

/lgtm
/approve

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