Revise Jekyll's incremental build

[szarnyasg]

Despite using incremental Jekyll builds, the local builds have gotten progressively slower in the last year, to the point where fixing a typo requires a ~25 second rebuild. We should look into this

[szarnyasg]

Rewriting the version selector logic made a big difference, hinting that there may be more performance gains to be made by optimizing the Liquid templates.