Setup Assistant: Support indented non-expandable steps

[interactivellama]

Currently if you mix expandable and non-exandable steps the media figures on the left are mis-aligned.

There are few options. The easiest is to follow the lead of Tree component and hide a button

[kwilloughby-sf]

@interactivellama, I spoke with Alan and he said we should use a Token so the media figures are aligned vertically. It appears the $var-spacing-horizontal-large Token which is 24px will do the trick.

He asked me to create a story to have some additional text added to the Setup Assistant page on the SLDS website.