Add issue templates for the pipeline execution team

What does this MR do and why?

Adding specific issue templates to simplify issue creation within the PE team and for consistency.

• Issue Implementation template -> a template to provide consistent formatting when splitting issues.
• Refinement Spike template -> used when creating a spike to extend the refinement process for more complex or poorly understood issues

Related to handbook MR: gitlab-com/www-gitlab-com!126044 (merged)

MR acceptance checklist

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