Add support for child themes and nested layouts

[LukeTowers]

Refs octobercms/october#2886 & octobercms/docs@42d7837

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[github-actions]

This issue will be closed and archived in 3 days, as there has been no activity in the last 60 days.
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[github-actions]

This issue will be closed and archived in 3 days, as there has been no activity in the last 60 days.
If this issue is still relevant or you would like to see it actioned, please respond and we will re-open this issue.
If this issue is critical to your business, consider joining the Premium Support Program where a Service Level Agreement is offered.

[github-actions]

This issue will be closed and archived in 3 days, as there has been no activity in the last 60 days.
If this issue is still relevant or you would like to see it actioned, please respond and we will re-open this issue.
If this issue is critical to your business, consider joining the Premium Support Program where a Service Level Agreement is offered.