a “cheap” way to support adding data to pages created through `proxy` or `page` by adding frontmatter at the expected destination url.
for example proxying a page to `/proxied.html`, by adding a sidecar frontmatter file at `source/proxied.html.frontmatter` (or source/proxied.frontmatter if directory indexes is activated) the frontmatter will be merged with the proxy source and available to `current_resource` at the destination.
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