Improve RDoc pages heading levels order #960

Job	Run time
lint	13s
ruby-versions / Generate Ruby versions	3s
test (3.0, ubuntu-latest)	27s
test (3.0, macos-latest)	28s
test (3.0, windows-latest)	1m 28s
test (3.1, ubuntu-latest)	24s
test (3.1, macos-latest)	19s
test (3.1, windows-latest)	2m 40s
test (3.2, ubuntu-latest)	22s
test (3.2, macos-latest)	20s
test (3.2, windows-latest)	3m 48s
test (3.3, ubuntu-latest)	27s
test (3.3, macos-latest)	30s
test (3.3, windows-latest)	3m 49s
test (head, ubuntu-latest)	31s
test (head, macos-latest)	24s
test (head, windows-latest)	2m 17s
test (jruby, ubuntu-latest)	2m 3s
test (jruby, macos-latest)	2m 7s
test (jruby-head, ubuntu-latest)	2m 9s
test (jruby-head, macos-latest)	1m 51s
test (truffleruby, ubuntu-latest)	2m 26s
test (truffleruby, macos-latest)	1m 58s
test (truffleruby-head, ubuntu-latest)	3m 11s
test (truffleruby-head, macos-latest)	3m 7s
	37m 22s

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