manual: Add workaround for too old Emacs on GNU ELPA build server

Emacs (and thus Org) on GNU ELPA build server is too old, so we have
to allow Org to continue when it encounters broken links.  Users will
notice these broken links, but this way we at least get to distribute
the Magit manual.

The broken links are those that link to Git's man pages.

docs/magit.org

@@ -11,6 +11,10 @@
 
 #+setupfile: .orgconfig
 
+# Emacs on GNU ELPA build server is too old, so all "man:" links
+# will be broken, but this at least allows us to generate the manual.
+#+options: broken-links:mark
+
 Magit is an interface to the version control system Git, implemented
 as an Emacs package.  Magit aspires to be a complete Git porcelain.
 While we cannot (yet) claim that Magit wraps and improves upon each