doc: Add architecture note for DefinitionBuilder and AncestorBuilder #3848

Job	Run time
windows (windows-2019, ucrt)	1m 4s
test (master-nightly-focal, test)	2m 17s
windows (windows-2019, mswin)	1m 30s
test (master-nightly-focal, stdlib_test)	3m 1s
windows (windows-2022, ucrt)	1m 5s
test (master-nightly-focal, rubocop validate test_doc build test_generate_stdlib)	2m 33s
windows (windows-2022, mswin)	1m 34s
test (master-nightly-focal, lexer confirm_lexer)	4m 2s
test (master-nightly-focal, lexer compile)	4m 2s
test (3.2-dev-focal, test)	2m 32s
test (3.2-dev-focal, stdlib_test)	2m 51s
test (3.2-dev-focal, rubocop validate test_doc build test_generate_stdlib)	2m 16s
test (3.2-dev-focal, lexer confirm_lexer)	4m 55s
test (3.2-dev-focal, lexer compile)	4m 9s
test (3.1-dev-focal, test)	2m 18s
test (3.1-dev-focal, rubocop validate test_doc build test_generate_stdlib)	2m 27s
test (3.1-dev-focal, lexer confirm_lexer)	4m 54s
test (3.1-dev-focal, lexer compile)	4m 53s
test (3.0-dev-focal, test)	2m 17s
test (3.0-dev-focal, rubocop validate test_doc build test_generate_stdlib)	3m 2s
test (3.0-dev-focal, lexer confirm_lexer)	4m 6s
test (3.0-dev-focal, lexer compile)	4m 9s
	1h 5m 57s

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