chore: add categorization labels to PR template

[petersutter]

How to categorize this PR?

/area dev-productivity
/area open-source
/kind enhancement

What this PR does / why we need it:

• Adds /area and /kind categorization commands to the PR template to help the community triage and organize contributions

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

NONE

[gardener-prow]

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