ruby: Make docs generation deterministic

The presence or lack of nroff on the host was changing the doc type. Set it
explicitly to be deterministic and reproducible.

(From OE-Core rev: dd857d2519fd4f38c67a6fa0087f72798166467a)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit f5053abb8957acf358b518ee3c76146dc5f4eb6c)
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/recipes-devtools/ruby/ruby_3.1.3.bb

@@ -76,6 +76,7 @@ EXTRA_OECONF = "\
     --enable-load-relative \
     --with-pkg-config=pkg-config \
     --with-static-linked-ext \
+    --with-mantype=man \
 "
 
 EXTRA_OECONF:append:libc-musl = "\