Extract appendix from main index.adoc

Extract the appendix into a separate asciidoc file in order to make the
main index.adoc easier to work with.
Remove the What's new in Spring 3 from docs

Remove the entire What's new in Spring 3 section from the reference
documentation.
Clean reference docs and prepare for Spring 4

Polish and cleanup the asciidoc source and prepare the reference guide
for the upcoming Spring 4.0 release.
Clean MVC reference documentation

Remove reference to removed deprecations and generally cleanup the MVC
documentation.
Cleanup Struts
Add WebSocket reference documentation

Update reference documentation with a new section for Spring 4's
WebSocket support.
Document MVC concepts introduced in Spring 4.0

Update the MVC reference documentation with the new concepts and classes
introduced in Spring 4.0
This reverts commit 6f874d734a2a38431c74444894f54797ecf74933.
Add What's new in 4.0 to reference documentation

Update the reference documentation with "What's new in Spring 4.0".