docs: improve docs for KeptnEvaluationDefinition

[thisthat]

Fixes #1334

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[sonarcloud]

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[StackScribe]

This conflicts with #1224:

• 1224 deletes the concepts/evaluations page
• Replaces it with the yaml-crd-ref/evaluationdefinition.md and yaml-crd-ref/evaluationprovider.md files, which have been updated for the v1alpha3 information.
• The content you added in lines 37-39 was not in 1224 but is now added
• I also added the example yaml file in the "Examples" section of the page.

Additional information in #1317 and #1337 . Those are still works in progress but eventually I'll be sure that salient information from those getting started guides are incorporated into the yaml-crd-ref and implementing files.

I think this PR should be closed and we should use the version in 1224