doc: change the canonical url to v1 instead of stable.

[fredrikekre]

Ref #26825

Should not be merged until we have made some changes to the gh-pages branch.

cc @mortenpi

[mortenpi]

Could you add the following to make.jl? This should ensure that the master generates the correct version selector.

# This overloads the function in Documenter that generates versions.js, to include
# v1/ in the version selector, instead of stable/.
#
# The function is identical to the version found in Documenter v0.19.6, except for
# the first line in the first for loop, which now include "v1" instead of "stable".
#
# Original:
# https://github.com/JuliaDocs/Documenter.jl/blob/v0.19.6/src/Writers/HTMLWriter.jl#L481-L506
#
import Documenter.Writers.HTMLWriter: generate_version_file
function generate_version_file(dir::AbstractString)
    named_folders = []
    release_folders = []
    tag_folders = []
    for each in readdir(dir)
        each in ("v1", "latest")             ? push!(named_folders,   each) :
        occursin(r"release\-\d+\.\d+", each) ? push!(release_folders, each) :
        occursin(Base.VERSION_REGEX, each)   ? push!(tag_folders,     each) : nothing
    end
    # put stable before latest
    sort!(named_folders, rev = true)
    # sort tags by version number
    sort!(tag_folders, lt = (x, y) -> VersionNumber(x) < VersionNumber(y), rev = true)
    # sort release- folders by version number
    vnum(x) = VersionNumber(match(r"release\-(\d+\.\d+)", x)[1])
    sort!(release_folders, lt = (x, y) -> vnum(x) < vnum(y), rev = true)
    open(joinpath(dir, "versions.js"), "w") do buf
        println(buf, "var DOC_VERSIONS = [")
        for group in (named_folders, release_folders, tag_folders)
            for folder in group
                println(buf, "  \"", folder, "\",")
            end
        end
        println(buf, "];")
    end
end

[mortenpi]

If we want, we could actually change the latest right away as well..

[mortenpi]

LGTM