message: docs: Add catalog-client to project structure
diff: @@ -89,6 +89,10 @@ are separated out into their own folder, see further down.
  There are no "core" packages in the backend. Instead we have `backend-common`
  which contains helper middleware and other utils.
  +- [`catalog-client`](https://github.com/backstage/backstage/tree/master/packages/catalog-client) -
  + A client to communicate with Backstage software catalog. Recommended for backend plugins, compatible
  + but not recommended for frontend plugins (use `@backstage/plugin-catalog` package instead).
  +
  - [`catalog-model/`](https://github.com/backstage/backstage/tree/master/packages/catalog-model) -
  You can consider this to be a library for working with the catalog of sorts.
  It contains the definition of an
  