docs: add info to implementing/tasks about sequential execution

[StackScribe]

Closes #1949

Draft status while I do some rewriting in response to Adam's remarks.

Answers the question: "Is there a way to define a given order for execution post-deployment tasks in Keptn KLT?"

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[agardnerIT]

The wording here confuses me.

To the question: "Can KeptnTasks be executed in sequential order", the text first essentially says: "No":

All KeptnTaskDefinition resources at the same level
(either pre-deployment or post-deployment) execute in parallel.

That's very clear.

But then towards the end of the page:

Perhaps you need to run a set of integration tests, a set of performance tests, and a set of regression tests.

Yes, perfect. Let's say I need to run integration tests first then regression (and only if they pass) performance tests.

Define one KeptnTaskDefinition resource that runs
integration tests, then regression test, then performance tests
in order.

So, I seem to have two answers: I cannot execute sequentially, but also, I can. Which is it?

[StackScribe]

@agardnerIT Thanks for this excellent critique. I did a rewrite of all of this -- see if it makes more sense now. The answer is that all KeptnTask resources that are defined as annotations to the KeptnApp resource for a specific stage execute in parallel but one KeptnTask resource can run many executables in order. Moreover, you can define a "parent" KeptnTaskDefinition that uses the functionRef syntax to run other KeptnTask resources in sequence, but only the parent KeptnTaskDefinition is specified in the KeptnApp annotations.

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