Change the current About page to an introduction page for osm-ng project

[honnip]

[Zaczero]

I've always thought having a clean, dedicated about page would be nice. The GitHub README isn't easily accessible to less technical users. I don't think replacing the existing one would work (because we need it for testing, and it's not publicly discoverable), but perhaps in addition to it. I think we could do something like this once there's a system in place for easy editing of the website. In the future, projects like OpenHistoricalMap or other alternative maps would be able to make use of it as well.