robots.txt in git and online are not the same

[Mark Polesky]

For some reason, http://www.lilypond.org/robots.txt differs
from Documentation/web/server/robots.txt as found in git.  
It looks as if it isn't being updated on the server?  I 
don't get it, but it's worth looking into, since this helps
remove outdated links when someone googles us from the
outside.

- Mark