Link to Atlas docs from Faust & ACM docs

[Kellen Mace]

We want Faust and ACM to both be "on ramps" for getting developers using Atlas. Currently though, there's isn't a top-level link developers can click to be sent to the Atlas docs.

Let's add a top-level link in the sidebar of both the Faust and ACM docs that resembles the image below and points to https://developers.wpengine.com/docs/node-engine/introduction.

Then developers who are reading the Faust or ACM docs have an easy way to navigate to the Atlas docs.