docs: note about existing content at same path on different branches

vintro

2023-08-14 f1c9ca495e450ecb62dade70c4b60d86e106f79c

 quartz/plugins/emitters/folderPage.tsx

@@ -74,9 +74,7 @@
        }

        const content = renderPage(slug, componentData, opts, externalResources)

        const fp = (file.data.slug! + ".html") as FilePath
        await emit({
        const fp = await emit({
          content,
          slug: file.data.slug!,
          ext: ".html",