docs: "Writing generators" code example + add source_url to enable li… #104

Job	Run time
ash-ci / mix deps.get	26s
ash-ci / audit	50s
ash-ci / MIX_ENV=test mix.compile	28s
ash-ci / MIX_ENV=dev mix.compile	20s
ash-ci / mix docs	34s
ash-ci / mix credo --strict	4s
ash-ci / mix format --check-formatted	26s
ash-ci / mix sobelow	22s
ash-ci / mix dialyzer	2m 11s
ash-ci / mix spark.cheat_sheets --check	11s
ash-ci / mix spark.formatter --check	4s
ash-ci / mix test	49s
ash-ci / mix deps.unlock --check-unused	30s
ash-ci / Deploy docs to GitHub pages.	9s
ash-ci / Release	0s
	7m 24s

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