Include docs for in-progress release on docs.julialang.org

[ViralBShah]

Currently, we have 1.7 released, 1.8 branched but being prepared, and 1.9-DEV. docs.julialang.org shows only 1.7 and 1.9-dev. It would be really handy to have the in-progress release being prepared (1.8-dev) on docs.julialang.org as well, along with the PDF.

[fredrikekre]

Documentation for release candidates are shown (#36859). In any case, the best documentation for any Julia 1.X is really the one built from the master branch.

[ViralBShah]

I don't see it on docs.julialang.org. My main point is that it is confusing for people who may wonder why a particular release number is being skipped. I also sometimes want to check what the RC documentation looks like, but don't find it in the obvious place I look.

[fredrikekre]

There haven't been a rc for 1.8 yet.

[ViralBShah]

Oh, so we only do it for RCs and not betas and such. Would be nice to have the betas as well.

[fredrikekre]

Yea, shouldn't be difficult to modify the code from the PR I linked above.