docs(collections): explain updating collection structure (#10232)
* docs(collections): explain updating collection structure
Adds a subsection to the Collections guide explaining that state controlling which items exist must live above the collection component, because collections are built by parsing a hidden render of the JSX before the real content renders.
Closes #10199
* docs(collections): trim structure section to the problem and fix
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Co-authored-by: Patrick Wehbe <patrick.wehbe.applications@gmail.com>
Co-authored-by: Robert Snow <rsnow@adobe.com>