Link to Version 5 teaching materials.

1. Change navbar to point to new `./lessons.html` page.
2. Create `./lessons.html` with links to `v4` and `v5`.

Note that `v5` isn't in this repo - we'll submodule it (or whatever) once it settles down.
Merge pull request #457 from gvwilson/integrating-v5-material

Link to Version 5 teaching materials.