Improve links in docs

[eddyerburgh]

We need to improve the links in the docs.

Currently the generated links in the sidebar and the URL hashes are poor:

A H2 title:

is(selector)

The generated URL:

https://vue-test-utils.vuejs.org/api/wrapper/#is-selector

It would be good to customize the generated URL and sidebar URLs to just use the method name:

https://vue-test-utils.vuejs.org/api/wrapper/#is

[eddyerburgh]

I've created an issue for custom header links—vuejs/vuepress#679

[eddyerburgh]

This was fixed in #1099