Run template tests

Add a Makefile for Ruby/`bundle`/Jekyll commands that I always forget. #2267

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Run time

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Job	Run time
test ubuntu-latest py3.11	39s
test macos-latest py3.13	25s
test ubuntu-latest py3.12	44s
test macos-latest py3.11	36s
test windows-latest py3.12	1m 48s
test windows-latest py3.13	2m 24s
test macos-latest py3.12	36s
test windows-latest py3.11	2m 28s
test ubuntu-latest py3.13	42s
	10m 22s