Improve visibility of mailing lists and add "Forum" section

[neteler]

At time the visibility of our mailing list is rather limited, at the related Community page resides under "About" which isn't intuitive when searching for "Support" (which we don't have - unlike on https://old.grass.osgeo.org/support/).

I don't have any obvious idea yet how to address that (different menu entry?) - suggestions are welcome.

While at it, we may also add a section "Forum", with

• https://gis.stackexchange.com/questions/tagged/grass
• Nabble mailing list mirror: http://osgeo-org.1560.x6.nabble.com/GRASS-f3884508.html (hint: the recent nabble spam has be flagged here)
• MARC mailing list mirror: https://marc.info/?w=4&r=1&s=grass&q=l
• anything else?

And also create a "Commercial support" subpage, see existing issue #34

[ecodiv]

Why not a separate menu for communiteit or alternatively support? It is such an essential part, a key selling point.

[neteler]

I much like the idea to promote "Community" to the top level menu!

[petrasovaa]

This seems to have been addressed by #369.
If there is anything left, open a separate issue.