Cleanup doc code

[mvandeberg]

Most documentation snippets and examples contain the same header. The reduce repeated code, this should be extracted into a shared header.

The namespace declaration is:

namespace capy = boost::capy;
using namespace boost::capy;

The using supersedes the namespace alias. Both is not needed. Instead, we should just use the namsespace alias and update all examples to explicitly prepend capy:: to clarify what symbols in examples are from capy.