Move AGI command 'gosub' static documentation to XML.

Move AGI command 'gosub' statis documentation to the new AstXML form.

(issue #15245)
Reported by: eliel
Patches:
      app_stack_static_conversion.txt uploaded by lmadsen (license 10)
      (with minor changes by me)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@199446 65c4cc65-6c06-0410-ace0-fbb531ad65f3