Port web content from opensolid.github.io

[ianmackenzie]

All images and interactive examples from opensolid.github.io should be moved to ianmackenzie.github.io/elm-geometry. As suggested in opensolid/opensolid.github.io#1, this move should probably also involve figuring out a structure that allows for a simple home page that links to the various examples, future guide documents etc. and generally makes the various contents more discoverable.