docs: improve how-to and ref info for apps

[StackScribe]

Replaces #1785 . That PR had lots of reviews and discussions but got corrupted.

Closes #1732

• This PR corrects and clarifies info about integrating KLT into a Kubernetes workflow/cluster
• Also adds remarks about annotations vs labels
• This PR includes an architecture piece about KeptnApp and KeptnWorkload but we decided that this belongs in the section about the Scheduler (docs: keptn Scheduler architecture documentation #1777). This file is temporarily still here until Yash can grab it.
• Information about the annotations for pre/post-deployment evaluations/tasks really is not part of Integrating the app. We probably need a separate page about using the release lifecycle management feature and move that information over there. For now, let's leave the info here and I filed create page about implementing the release lifecycle management feature #1784 .

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[odubajDT]

Nice work :)