Add RDoc::Server and --server flag #869

Job	Run time
lint	15s
ruby-versions / Generate Ruby versions	4s
test (3.0, ubuntu-latest)	19s
test (3.0, macos-latest)	25s
test (3.0, windows-latest)	1m 18s
test (3.1, ubuntu-latest)	23s
test (3.1, macos-latest)	24s
test (3.1, windows-latest)	1m 16s
test (3.2, ubuntu-latest)	24s
test (3.2, macos-latest)	21s
test (3.2, windows-latest)	1m 26s
test (3.3, ubuntu-latest)	20s
test (3.3, macos-latest)	17s
test (3.3, windows-latest)	2m 37s
test (head, ubuntu-latest)	19s
test (head, macos-latest)	19s
test (head, windows-latest)	1m 13s
test (jruby, ubuntu-latest)	1m 31s
test (jruby, macos-latest)	1m 8s
test (jruby-head, ubuntu-latest)	1m 28s
test (jruby-head, macos-latest)	1m 2s
test (truffleruby, ubuntu-latest)	1m 31s
test (truffleruby, macos-latest)	1m 36s
test (truffleruby-head, ubuntu-latest)	1m 58s
test (truffleruby-head, macos-latest)	1m 16s
	23m 10s

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