Remove User Manual from Doxygen documentation

Remove the version of the user manual published at
http://cprover.diffblue.com/cbmc-user-manual.html
and link to http://www.cprover.org/cprover-manual/ instead, in order to
make a clearer separation between the _user_ and _developer_
documentation.

There is updated source intended for the user manual, with almost
identical contents to the removed cbmc-user-manual.md, in the
doc/cprover-manual folder.

Author: John Nonweiler

doc/architectural/cbmc-architecture.md

@@ -41,10 +41,10 @@ digraph G {
 }
 \enddot
 
-The \ref cbmc-user-manual "CBMC User Manual" describes CBMC from a user
-perspective. Each node in the diagram above links to the appropriate
-class or module documentation, describing that particular stage in the
-CBMC pipeline.
+The [CPROVER User Manual](http://www.cprover.org/cprover-manual/) describes
+CBMC from a user perspective. Each node in the diagram above links to the
+appropriate class or module documentation, describing that particular stage
+in the CBMC pipeline.
 CPROVER is structured in a similar fashion to a compiler. It has
 language specific front-ends which perform limited syntactic analysis
 and then convert to an intermediate format. The intermediate format can

doc/architectural/front-page.md

@@ -57,8 +57,8 @@ Overview of Documentation
 
 ### For users:
 
-* The \ref cbmc-user-manual "CBMC User Manual" details the capabilities of
-  CBMC and describes how to install and use these tools. It
+* The [CPROVER User Manual](http://www.cprover.org/cprover-manual/) details the
+  capabilities of CBMC and describes how to install and use these tools. It
   also covers the underlying theory and prerequisite concepts behind how
   these tools work.