Add a latest route for permalinking

[HCLarsen]

If I were to create a link to the docs from a blog post or static site, it links to a specific version. Unlike the Crystal stdlib docs, I don't see a way to link to the newest version of the docs, instead of a specific static version. This could be problematic if I were to link to them from a static website.

[nobodywasishere]

Ah actually this is already supported, but I guess not well documented. You can link to either crystaldoc.info/service/username/project or crystaldoc.info/service/username/project/latest and they'll point to the latest non-nightly tag that successfully built