Generate relative URLs for user docs index page

[shhyou]

This PR allows the html renderer to generate relative URLs for references of the installation-scope tags from the user-scope documents (in (find-user-doc-dir) and (find-user-pkgs-dir)). In particular, the index page of the user doc ((find-user-doc-dir)/index.html) will link to all packages using relative URLs.

I am not sure how to approach this problem differently.

• [ ] Make the relative path option configurable
• [ ] Documentation
• [ ] Check the layered-installations
• [ ] (Tests?)