add info on how to cross-reference gallery examples

docs/developers/contributing/documentation/index.md

@@ -577,3 +577,10 @@ is not necessary—you can copy-paste the code above it. However, it is required
 Because our example gallery is built from Python scripts, you need to ensure this
 block is present in all contributed examples.
 ````
+
+### Cross-referencing Gallery examples
+
+If you want to generate links to Gallery examples from anywhere in the docs, then the cross-referencing format you use will depend on the format of the doc you are writing. Note that the gallery examples live in `/gallery` despite being in `napari/examples` because [`docs/docs/conf.py`](https://github.com/napari/docs/blob/main/docs/conf.py) specificies that examples are built into the `gallery` directory. The [Sphinx cross-reference namespace](https://sphinx-gallery.github.io/stable/advanced.html#know-your-gallery-files) is generated with the `sphx_glr` prefix, then path separators are converted to underscores, for example `/gallery/add_image.py` becomes `_gallery_add_image.py` to get the end result `sphx_glr_gallery_add_image.py`. 
+
+- For `.md` files (myst, used in the majority of docs), use the `{ref}` directive. ``{ref}`sphx_glr_gallery_add_image.py` `` displays as: {ref}`sphx_glr_gallery_add_image.py`
+- For `.py` files (rst, used in the example gallery), cross-reference with the `:ref:` directive: `` :ref:`sphx_glr_gallery_example.py` ``.