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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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