docs: Add Notifier.PubSub example #3989

Job	Run time
ash-ci (SimpleSat) / audit	1m 12s
ash-ci (SimpleSat) / mix igniter.upgrade	4s
ash-ci (SimpleSat) / mix deps.get	15s
ash-ci (Picosat) / audit	56s
ash-ci (Picosat) / mix deps.get	17s
ash-ci (Picosat) / mix igniter.upgrade	3s
ash-ci (SimpleSat) / MIX_ENV=test mix.compile	59s
ash-ci (Picosat) / MIX_ENV=test mix.compile	57s
ash-ci (Picosat) / mix format --check-formatted	1m 1s
ash-ci (Picosat) / mix docs	1m 7s
ash-ci (Picosat) / mix spark.cheat_sheets --check	56s
ash-ci (Picosat) / mix spark.formatter --check	58s
ash-ci (Picosat) / Deploy docs to GitHub pages.	0s
ash-ci (Picosat) / MIX_ENV=dev mix.compile	49s
ash-ci (Picosat) / mix ash.codegen --check	0s
ash-ci (Picosat) / mix credo --strict	1m 3s
ash-ci (Picosat) / mix dialyzer	4m 0s
ash-ci (Picosat) / mix sobelow	1m 3s
ash-ci (Picosat) / mix test	2m 27s
ash-ci (Picosat) / mix deps.unlock --check-unused	48s
ash-ci (SimpleSat) / mix docs	1m 4s
ash-ci (SimpleSat) / mix format --check-formatted	51s
ash-ci (SimpleSat) / mix credo --strict	1m 4s
ash-ci (SimpleSat) / mix sobelow	59s
ash-ci (SimpleSat) / Deploy docs to GitHub pages.	0s
ash-ci (SimpleSat) / mix spark.cheat_sheets --check	51s
ash-ci (SimpleSat) / mix spark.formatter --check	48s
ash-ci (SimpleSat) / MIX_ENV=dev mix.compile	55s
ash-ci (SimpleSat) / mix dialyzer	3m 38s
ash-ci (SimpleSat) / mix test	2m 37s
ash-ci (SimpleSat) / mix ash.codegen --check	0s
ash-ci (SimpleSat) / mix deps.unlock --check-unused	49s
ash-ci (Picosat) / Release	0s
ash-ci (SimpleSat) / Release	1s
	32m 32s

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