docs: Added documentation for usage of spot instances

[Ankitasw]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
This PR adds the documentation for usage of spot instances in CAPA for AWS Machines.

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 #3246

Checklist:

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

Release note:

Added documentation for usage of spot instances for AWS Machines.

[k8s-ci-robot]

@Ankitasw: This issue is currently awaiting triage.

If CAPA/CAPI contributors determines this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.

The triage/accepted label can be added by org members by writing /triage accepted in a comment.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[sedefsavas]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sedefsavas

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [sedefsavas]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment