Document `konveyor.io` namespace labels and best practices

There are `konveyor.io` labels used across the default rulesets for source/target usage within the project. These labels need to be clearly documented for their purpose and we should advocate that custom rules authors do not use these labels to avoid confusion with some of the workflows we have in the current user experience.

Start by reviewing the default rulesets and locating the existing `konveyor.io` labels and the workflows that use them. Document each label's purpose and explain why custom rules authors should avoid the namespace; done means the guidance covers source/target usage and the relevant user-experience confusion.

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