Introduction docs link 404

[joshmgross]

Problem

In the introduction, there are links to Custom Elements, Actions, and Targets.

All of these links 404 (ex. https://github.github.io/guide/custom-elements)

Expected behavior

These links should go to the correct pages:
https://github.github.io/catalyst/guide/targets/
https://github.github.io/catalyst/guide/actions/

(There doesn't appear to be a Custom Elements page in the docs)