Address confusion with links to old package versions

[milesfrain]

There are lots of links in docs pointing to old package versions on pursuit, and this leads to confusion. Some ideas to improve this situation are discussed in this Discourse thread and include:

• Replace the version number in the url with latest (unless explicitly linking to a version). This is what Elm does.
• Add a visual indicator when not browsing the latest version. Either a floating header or background.