adjust docs/make.jl to changes in Documenter.jl

[ThomasBreuer]

Now we can almost process the documentation with Documenter.jl 1.0.

• The creation of hyperlinks to the GAP documentation works again.
• The errors because of docstrings that are not referenced in docs/src/*.md are gone because the docstrings are now referenced. (I do not like this at all. Why is it an error not to include all available docstrings?)
• There are still errors because three explicit cross-references to the JuliaInterface manual (in the directory assets/html/JuliaInterface) are not accepted because they are local links outside the build directory.

[codecov]

Codecov Report

Merging #945 (e336b58) into master (3cfb342) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #945   +/-   ##
=======================================
  Coverage   75.70%   75.70%           
=======================================
  Files          51       51           
  Lines        4169     4169           
=======================================
  Hits         3156     3156           
  Misses       1013     1013           

[fingolfin]

You can also add checkdocs = :none to the arguments of makedocs to stop it from throwing an error when a docstring does not appear in the manual.

Alternatively one can set warnonly = true -- this is actually what we had before (warnonly replaces the former option strict which defaulted to false -- so in a sense they just toggled the default value...). The drawback is that warnonly = true will also treat other things as only warnings, were we really might want to get an error, so we notice it and can fix it....

[fingolfin]

Oops, I kinda forgot about this PR.

Everything looks good, tests pass -- is there any reason not to merge it?

[fingolfin]

Oh wait, now I understand: the three errors you are referring to are for when one switches to actually using Documenter.jl 1.x -- which this PR does not, it just prepares for this.