Add documentation that explains about use of deleteCRDResources

[sbulage]

QA Observations:

Followed below scenario's considering the deleteCRDResources values and annotation behavior.

Scenario 1

• Create a GitRepo which installs CRD's along with the application.(Used this fleet.yaml file.)
• Make sure deleteCRDResources: true must be present in the fleet.yaml.
• Verified that annotation helm.sh/resource-policy: keep is absent.

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Scenario 2

• Create a GitRepo which installs CRD's along with the application.(Used this fleet.yaml file.)
• Update the value of deleteCRDResources: false in the fleet.yaml.
• Verified that annotation helm.sh/resource-policy: keep is present.

Consider this conclusion for documentation.

Conclusion:

If we don't want to see annotation helm.sh/resource-policy: keep then we have to use the deleteCRDResource option with true value in fleet.yaml.

Originally posted by @sbulage in #2860 (comment)