Stop using linkcheck.

[oxinabox]

Documenter marks linkcheck as experimental, and has it disabled by default.
Given removing it will just fix #101 and let us stop wasting time on it, not using an experimental feature seems like a good idea.

https://juliadocs.github.io/Documenter.jl/stable/lib/public/#Documenter.makedocs

Experimental keywords

In addition to standard arguments there is a set of non-finalized experimental keyword arguments. The behaviour of these may change or they may be removed without deprecation when a minor version changes (i.e. except in patch releases).

linkcheck – if set to true makedocs uses curl to check the status codes of external-pointing links, to make sure that they are up-to-date. The links and their status codes are printed to the standard output. If strict is also enabled then the build will fail if there are any broken (400+ status code) links. Default: false.

linkcheck_ignore allows certain URLs to be ignored in linkcheck. The values should be a list of strings (which get matched exactly) or Regex objects. By default nothing is ignored.

linkcheck_timeout configures how long curl waits (in seconds) for a link request to return a response before giving up. The default is 10 seconds.

Alternatively we could use linkcheck_ignore to just ingore the codecov one that keeps causing problems, or try a longer timeout.

[BSnelling]

This seems like a good choice for the moment as linkcheck is categorised as experimental by Documenter.