docs: Add code blocks/syntax highlighting for examples in `AshJsonApi.Test` #958

Job	Run time
ash-ci / audit	37s
ash-ci / mix deps.get	13s
ash-ci / mix igniter.upgrade	3s
ash-ci / MIX_ENV=test mix.compile	37s
ash-ci / mix spark.cheat_sheets --check	27s
ash-ci / mix docs	1m 50s
ash-ci / mix credo --strict	38s
ash-ci / mix dialyzer	3m 46s
ash-ci / MIX_ENV=dev mix.compile	34s
ash-ci / mix format --check-formatted	25s
ash-ci / mix sobelow	28s
ash-ci / mix spark.formatter --check	35s
ash-ci / mix test	1m 3s
ash-ci / mix deps.unlock --check-unused	24s
ash-ci / mix ash.codegen --check	0s
ash-ci / Deploy docs to GitHub pages.	0s
ash-ci / NIF ${{ matrix.nif }} - ${{ matrix.job.target }} (${{ matrix.job.os }})	0s
ash-ci / Release	0s
	11m 40s

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