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

[ci skip]

Signed-off-by: Anatoly Borodin <anatoly.borodin@gmail.com>