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.
**Note**: the generated files contains raw text and not html for now, and they are displayed as a raw textarea on the website.
This not the final step and in the end the files will be real html. This will be done in a future diff.