[CMAKE] Install the man pages

Summary:
This adds support for the man pages installation.
The last directory is hardcoded to man1. This is correct as long as
the man files are hardcoded to section 1 only.

Test Plan:
For all platforms:

mkdir -p buildcmake/install && cd install
cmake -GNinja .. -DCMAKE_INSTALL_PREFIX="install" # Other options

depending on the platform

ninja
ninja install

Check that the man pages are installed under install/share/man/man1.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4147