[chore:project] Add development planning files to `.gitignore` #39

Job	Run time
validate-and-test (ubuntu-latest, 3.11)	8s
validate-and-test (ubuntu-latest, 3.12)	11s
validate-and-test (windows-latest, 3.11)	34s
validate-and-test (windows-latest, 3.12)	30s
	1m 23s

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