[DOC] Rename doc/*.rb as doc/*.rdoc

With `:markup: ruby` directive so that they are parsed as ruby
scripts.

doc/string.rdoc

@@ -1,3 +1,5 @@
+# :markup: ruby
+
 class String
   # call-seq:
   #   String.new(string = '') -> new_string

doc/transcode.rdoc

@@ -1,3 +1,5 @@
+# :markup: ruby
+
 class String
   # call-seq:
   #   encode(dst_encoding = Encoding.default_internal, **enc_opts) -> string