Remove repo argument to makedocs

[gdalle]

Avoids the following warning by letting Documenter infer the repository instead of giving repo::String:

┌ Warning: Unable to determine the repository root URL for the navbar link.
│ This can happen when a string is passed to the `repo` keyword of `makedocs`.
│ 
│ To remove this warning, either pass a Remotes.Remote object to `repo` to completely
│ specify the remote repository, or explicitly set the remote URL by setting `repolink`
│ via `makedocs(format = HTML(repolink = "..."), ...)`.
└ @ Documenter.HTMLWriter ~/.julia/packages/Documenter/1HwWe/src/html/HTMLWriter.jl:707

The alternative would be to provide an explicit Remotes.GitHub, Remotes.GitLab or Remotes.URL depending on the host (see the release notes https://documenter.juliadocs.org/v1.0.0/release-notes/).
Not sure which is best.

Is this enough to fix #430?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e55f550) 93.80% compared to head (4f2af8f) 93.80%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #450   +/-   ##
=======================================
  Coverage   93.80%   93.80%           
=======================================
  Files          24       24           
  Lines         742      742           
=======================================
  Hits          696      696           
  Misses         46       46           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[oxinabox]

Makes sense

[goerz]

LGTM!