doc: import AnchorJS 5.0.0 (minified script)

https://www.bryanbraun.com/anchorjs/

AnchorJS is under the MIT license.
This bundle is direct from:
https://github.com/bryanbraun/anchorjs/blob/14e912226316aea775d957fa96c295193157f31d/anchor.min.js

Doesn't seem much point in messing with NPM for this, it is a small and
fairly simple script.

(I unminfied and checked for signs of malicious content, it seems fine)

Author: johnbartholomew