Use relative links in `doc`, fix broken generated HTML links

Links like (doc/web_hooks/web_hooks.md) work in the GitLab source code
web interface, but the HTML generator produces broken links in the `doc`
subdirectories:  
http://doc.gitlab.com/ce/hooks/doc/web_hooks/web_hooks.html  
instead of the right one  
http://doc.gitlab.com/ce/web_hooks/web_hooks.html  
in  
http://doc.gitlab.com/ce/hooks/custom_hooks.html.

Fixes #14338 


See merge request !3261