Page - section grouping follow-ups

~~fix(page): fix section spacing #6766 (comment)~~
- ~~Remove any references for __main-list~~
~~fix(page): fix section spacing #6766 (comment)~~
- ~~Remove any references for __main-nav, and/or any sort of tertiary nav stuff (deprecated && removed)~~
fix(page): fix section spacing #6766 (review) and fix(page): fix section spacing #6766 (comment)
- Support__main-body as a child for all of the page sections except __main-group
- Probably just support pf-m-limit-width on all, too, except __main-width? I think there are only a couple of elements that don't support it, and they could for consistency.
- ~~Try keeping padding on the outer __main-[section type] and not moving it down into __main-body. Should allow us to remove some CSS that's in there currently.~~

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