Some links are broken

[Frenco]

The getFromLocation link in this https://docs.feathersjs.com/api/authentication/client.html#getaccesstoken is broken (well technically it doesn't have a link specified). There are quite a few of those in the documents, which could be found through a markdown linter.

[David Luecke]

There was a link checker added in #1284 but I believe it never actually ran during build partly because it did not work reliably with the new heading style (see #1336)

[David Luecke]

The new docs do have a link checker and all links are verified to be working now.