Add rdoc as an explicit dependency for Ruby 4.0 compatibility #698

Job	Run time
ci-data	4s
Compile native gem for aarch64-linux	6m 12s
Compile native gem for x86_64-linux	9m 22s
Compile native gem for arm64-darwin	6m 3s
Compile native gem for x86_64-darwin	5m 53s
Compile native gem for x86_64-linux-musl	5m 58s
Compile native gem for aarch64-linux-musl	6m 9s
Compile native gem for x64-mingw-ucrt	6m 5s
	45m 46s

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