feat(export): add metadata file to preserve page icons and ordering on import (#1877)

* feat(export): add metadata file to preserve page icons and ordering on import
- Export includes `docmost-metadata.json`
- Import reads metadata to restore icons and sort siblings by original position

* cleanup

* bonus fixes

* handle unknown prosemirror nodes

* add docmost app  version

apps/server/src/common/helpers/types/export-metadata.types.ts

@@ -0,0 +1,14 @@
+export type ExportPageMetadata = {
+  pageId: string;
+  slugId: string;
+  icon: string | null;
+  position: string;
+  parentPath: string | null;
+};
+
+export type ExportMetadata = {
+  exportedAt: string;
+  source: 'docmost';
+  version: string;
+  pages: Record<string, ExportPageMetadata>;
+};