cmake: Generate man pages at install time
This PR enables the man pages to be generated on the fly during installation.
As the GNU help2man tool is required to generate man pages, the INSTALL_MAN build option is disabled by default.
Addresses this comment:https://github.com/bitcoin/bitcoin/blob/809d7e763cc9bdfff3288860a1c530460c76ffff/src/CMakeLists.txt#L456