Add `:showdoc:` directive (RFC #11) #13082

Job	Run time
x86_64-gnu-test (1.7.3)	33m 9s
x86_64-gnu-test-preview_mt	6h 0m 13s
x86_64-gnu-test (1.8.2)	34m 42s
x86_64-gnu-test (1.9.2)	34m 23s
x86_64-musl-test	35m 43s
x86_64-gnu-test (1.10.1)	36m 42s
x86_64-gnu-test (1.11.2)	33m 33s
x86_64-gnu-test (1.12.2)	37m 24s
x86_64-gnu-test (1.13.3)	33m 54s
check_format	1m 39s
x86_64-gnu-test (1.14.0)	34m 48s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	33m 33s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 50s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	34m 35s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	34m 24s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	32m 35s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	33m 9s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	32m 3s
	15h 9m 19s

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