[DOC] Split the examples `StringIO` into the document file #526

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.0	18s
ubuntu-24.04 2.7	43s
ubuntu-24.04 3.2	22s
ubuntu-24.04 3.1	19s
ubuntu-24.04 head	14s
ubuntu-24.04 3.4	12s
ubuntu-22.04 2.7	43s
ubuntu-24.04 3.3	16s
ubuntu-22.04 debug	25s
ubuntu-22.04 3.2	23s
ubuntu-22.04 3.4	17s
ubuntu-24.04 debug	23s
ubuntu-22.04 3.0	15s
ubuntu-22.04 3.1	22s
ubuntu-22.04 head	22s
ubuntu-22.04 3.3	26s
ubuntu-24.04 truffleruby-head	26s
ubuntu-24.04 jruby-head	1m 5s
ubuntu-22.04 truffleruby-head	29s
ubuntu-22.04 jruby-head	1m 7s
	9m 11s

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