Add rbs support to tool/rdoc-srcdir for make html #12876

Job	Run time
ruby-bench (--warmup=1 --bench=1 --excludes=shipit)	13m 57s
make (test-bundler-parallel, 50)	20m 31s
make (check, i686)	11m 33s
make (check, --disable-yjit)	7m 45s
make (check, cppflags=-DVM_CHECK_MODE)	9m 8s
make (check, ubuntu-24.04, capi)	9m 57s
make (test-bundled-gems)	13m 23s
make (check, --enable-shared --enable-load-relative)	8m 8s
make-ibm	0s
Ubuntu result	3s
	1h 34m 25s

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