Add config.cascade

This commit adds support for using the `cascade` keyword in your configuration file(s), e.g. `config.toml`.

Note that

* Every feature of `cascade` is available, e.g. `_target` to target specific page sets.
* Pages, e.g. the home page, can overwrite the cascade defined in config.

Fixes #8741

hugolib/content_map_page.go

@@ -462,10 +462,13 @@ func (m *pageMap) assembleSections() error {
 
 		if parent != nil {
 			parentBucket = parent.p.bucket
+		} else if s == "/" {
+			parentBucket = m.s.siteBucket
 		}
 
 		kind := page.KindSection
 		if s == "/" {
+
 			kind = page.KindHome
 		}