packaging: Build manpage in separate directory to other documentation

We were installing two copies of the manpage:

* One useless one in the `share/doc` folder, because we copy the doc/
  folder into share/
* The one we deliberately installed into `share/man` etc

The solution is to build the manpage into the `manpage` directory when
building the bindist, and then just install it separately.

Fixes #23707