Documentation: Update PR Checklist Template (Edge Unit Tests and keywords/families)

[zdz2101]

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Other

Let us know where something needs a refresh or put your idea here!

Equivalent to pharmaverse/admiral#1331 but for admiraldev repo

PR checklist should include a nudge towards developers looking at edge cases in their tests, e.g. empty datasets, errors, etc.

PR checklist should also include a link to the list of approved keywords and families allowed in an admiral function. Developers and reviewers should adhere to this list. If a new one is needed, then a proposal should be made.