vim-patch:9.1.0402: filetype: mdd files detected as zsh filetype

Problem:  filetype: mdd files detected as zsh filetype
Solution: detect '*.mdd' files as sh filetype, add links
          to reference documentation (Wu, Zhenyu)

closes: vim/vim#14741

63f2a5b8ad

Co-authored-by: Wu, Zhenyu <wuzhenyu@ustc.edu>

runtime/lua/vim/filetype.lua

@@ -952,6 +952,7 @@ local extension = {
   env = detect.sh,
   ksh = detect.ksh,
   sh = detect.sh,
+  mdd = 'sh',
   sieve = 'sieve',
   siv = 'sieve',
   sig = detect.sig,
@@ -1208,7 +1209,6 @@ local extension = {
   zsh = 'zsh',
   zunit = 'zsh',
   ['zsh-theme'] = 'zsh',
-  mdd = 'zsh',
   vala = 'vala',
   web = detect.web,
   pl = detect.pl,