Updating Pipeline Execution process to include creating a spike for complex refinement.

Why is this change being made?

In a recent team discussion we talked about our refinement process and ways we could make it more clear what was expected. One of the items that came out of that discussion was to clarify that refinement is expected to be a timeboxed activity and that if it will require more effort then we should track that effort through a spike. This will allow us to account for the extra time during milestone planning, rather than it being hidden in the weighting process.

Related MR: gitlab-org/gitlab!123978 (merged)

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