docs: add polymorphic relationships guide #4157

Job	Run time
ash-ci (Picosat) / audit	51s
ash-ci (Picosat) / mix deps.get	13s
ash-ci (Picosat) / mix igniter.upgrade	0s
ash-ci (SimpleSat) / mix deps.get	38s
ash-ci (SimpleSat) / audit	1m 8s
ash-ci (SimpleSat) / mix igniter.upgrade	6s
ash-ci (Picosat) / MIX_ENV=test mix.compile	54s
ash-ci (SimpleSat) / MIX_ENV=test mix.compile	50s
ash-ci (Picosat) / mix spark.formatter --check	52s
ash-ci (Picosat) / mix format --check-formatted	48s
ash-ci (Picosat) / Deploy docs to GitHub pages.	4s
ash-ci (Picosat) / mix docs	55s
ash-ci (Picosat) / mix spark.cheat_sheets --check	57s
ash-ci (Picosat) / mix credo --strict	57s
ash-ci (Picosat) / mix sobelow	1m 0s
ash-ci (Picosat) / mix ash.codegen --check	0s
ash-ci (Picosat) / mix deps.unlock --check-unused	49s
ash-ci (Picosat) / mix test	2m 29s
ash-ci (Picosat) / MIX_ENV=dev mix.compile	40s
ash-ci (Picosat) / mix dialyzer	3m 44s
ash-ci (SimpleSat) / mix docs	54s
ash-ci (SimpleSat) / mix spark.formatter --check	57s
ash-ci (SimpleSat) / mix format --check-formatted	1m 4s
ash-ci (SimpleSat) / Deploy docs to GitHub pages.	8s
ash-ci (SimpleSat) / mix spark.cheat_sheets --check	58s
ash-ci (SimpleSat) / mix ash.codegen --check	0s
ash-ci (SimpleSat) / mix credo --strict	1m 13s
ash-ci (SimpleSat) / mix sobelow	49s
ash-ci (SimpleSat) / mix deps.unlock --check-unused	46s
ash-ci (SimpleSat) / MIX_ENV=dev mix.compile	39s
ash-ci (SimpleSat) / mix dialyzer	3m 18s
ash-ci (SimpleSat) / mix test	2m 17s
ash-ci (Picosat) / Release	0s
ash-ci (SimpleSat) / Release	1s
	30m 59s

Provide feedback