Add a help jumping-off page

This adds a simple jump off page to OSM's most prominent sources of help
and documentation -- the new welcome page, LearnOSM, help.osm.org, and
wiki.osm.org.

The latter two were available in the sidebar under "Help", but space there is limited, and having a dedicated page permits some explanatory text as well as the addition of the welcome page and LearnOSM.