Update getting started docs to show modules.d for enabling modules

[dedemorton]

In the getting started docs, we show the older config where you specify all the modules in the .yml file. This still works, but it's not ideal because it conflicts with the simpler way (modules.d) of enabling/disabling modules.

This topic needs to be updated to show the command for enabling/disabling modules, and it needs to point to the topic about specifying which modules to run.

[dedemorton]

Might be good to streamline the getting started experience and pull the conceptual info that's currently embedded into the GS into an overview. The current guide embeds a lot of conceptual content that makes the getting started experience seem harder than it actually is.

[dedemorton]

Closing because this is a duplicate of #5512, which is already fixed and merged into master.