add anchors to documentation website

[timmie]

I would love to see achors appearing at website of the GRASS docs once the user hovers the headline.

This way, we could easily reference sections of the docs in ML & support forums or even articles.

Example: Sphinx generated: ​http://pythonhosted.org/Whoosh/intro.html#introduction-to-whoosh

But Trac seems to have it too.

[neteler]

We use CMSMS. Hence, a volunteer would need to study ​http://docs.cmsmadesimple.org/tags/core/cms_selflink and related extensions.

[neteler]

We are recreating the entire site with "hugo", see here: ​​https://github.com/OSGeo/grass-website

Wishes pls post there, closing here (our CMSMS site will be deactivated soon).