add deprecation note to http://cocoon.apache.org/2.1/userdocs/xsp/index.html

[Gregor J. Rothfuss]

i would suggest to add a deprecation note to the XSP documentation and 
point to newer technologies.

it makes me squirm to see all these xsp questions by cocoon beginners 
who should proably use flow/cforms/jx

just a thought,
-gregor