add `html` and `install-html` target to make

[eugenwintersberger]

Currently the HTML documentation is not built by default in the all target. However, the install targets expects the documentation to be there. In order to solve this problem the standard GNU targets html and install-html should be implemented

• html builds the HTML documentation
• install-html installs the HTML documentation

The default targets for make along with their particular functions are documented in
https://www.gnu.org/prep/standards/html_node/Standard-Targets.html .