It seems since v6 is released that the v5 docs have disappeared - https://apple.github.io/foundationdb/ doesn’t seem to have it. This breaks a lot of the useful links in various places for newcomers.
I found the source here so that’s ok for me, https://github.com/apple/foundationdb/blob/release-5.2/documentation/Makefile but perhaps it would be better to have the generated older versions available online somewhere too?