Large site fixes

I ended up getting drawn into doing a copyediting and cleanup pass over the entire site.  This series:

- Cleans up some element spacing,
- Fixes quite a few formatting inconsistencies on the example pages,
- Improves a bunch of the wording,
- Slightly restructures the top-level sections on the front page, and
- Removes the download/install/license/etc. sections from the example pages (#2).

I may have inadvertently introduced a few technical errors along the way, but hopefully on balance this is an improvement.

So, presented for your comments: an unreviewable blob.