refactor: split `ak._do` into `meta` and `content` #4034

Job	Run time
Determine Pyodide Python version	6s
Build Python	18s
Build C++ x86	51s
Build C++ WASM	25s
Execute cppyy notebook	1m 44s
Build Docs	9m 41s
Deploy Branch Preview	1m 1s
Deploy	0s
	14m 6s

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