[CMAKE] Transform man pages for web rendering

Summary:
This diff adds a new install-manpages-html target which converts and
installs man pages as jekyll compliant files to the
<install_dir>/share/man/html directory.
This will be useful for integrating the manpages to the website.

Test Plan:
Should install and convert the man pages:

ninja install-manpages-html

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7249