Add current release candidate to the versions in the docs menu bar

[logankilpatrick]

I feel like I have opened this issue before, can someone refresh me on why 1.6.0-rc1 is not a docs option here: https://docs.julialang.org/en/v1/ but https://docs.julialang.org/en/v1.6-dev/ works?

[logankilpatrick]

This:

julia/doc/make.jl

Line 320 in 7853ddd

# Hack to make rc docs visible in the version selector

makes me think perhaps it's a technical limitation?

[fredrikekre]

Fixed by #36859