Support Documenter.jl v1.0

[gdalle]

I think it would be great to make v1.0 of Documenter.jl the default in PkgTemplates

There are a few things in the docs/make.jl that no longer seem necessary when I start building existing docs with v1.0, but I'm not very confident to make these changes myself in the template.

Furthermore, the new version is not compatible with Julia < 1.6, so that's something to keep in mind.

See also:

• Announcement: https://discourse.julialang.org/t/ann-documenter-1-0/103888
• Release notes: https://documenter.juliadocs.org/v1.0.0/release-notes/