add section on C23doc/manual/R-exts.texi #8053

Job	Run time
MacOS (macOS-13)	53m 1s
Windows (check)	50m 56s
MacOS (macOS-14)	24m 5s
Linux (gcc, debian:stable, intel)	33m 34s
Windows (installer)	20m 0s
Linux (gcc, debian:stable, arm)	30m 8s
Linux (gcc, debian:testing, intel)	33m 46s
Linux (gcc, debian:testing, arm)	30m 59s
Linux (clang, debian:stable, intel)	33m 58s
Linux (clang, debian:stable, arm)	30m 13s
Linux (clang, debian:testing, intel)	36m 27s
Linux (clang, debian:testing, arm)	30m 43s
	6h 47m 50s

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