Update docs build

[ajwheeler]

This PR:

• deletes test/Manifest.toml, which I think is being tracked by mistake
• adds a compat entry for Documenter in the test Project.toml
• tweaks docs/make.jl to work with the newest Documenter versions
• Drops UNLIMITED and is_selection_valid from @docs blocks because they don't seem to be defined.
  • I don't understand what's going on with this one. I see them in the source, but they don't seem to be in the current docs, and I can't access them if I load the package myself.

edit: sorry for submitting something which fails, but I think this should be straightforward to fix for someone who understands what's going on with UNLIMITED and is_selection_valid.

[mkitti]

Thank you for the help and the hints for how to fix things. I'm heading in a slightly different direction with the size_threshold for now.

See #1172