DOCS: Adding manual assets docs (#916)

Co-authored-by: Chris Sewell <chrisj_sewell@hotmail.com>

docs/advanced/advanced.md

@@ -332,6 +332,43 @@ After the build, view the HTML with:
 
 `start docs\_build\html\index.html`
 
+(sphinx:manual-assets)=
+## Manually specify extra files/folders to be included in a website
+
+Jupyter Book will copy over any files that are linked from within its pages so that the links work in the built website.
+However, sometimes you'd like to manually ensure that files and folders are included in your built website.
+For example, if you'd like to link to them from *outside* your built documentation, but not from within your built documentation.
+
+To manually specify items to copy over, use the [`html_extra_path` Sphinx configuration](https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_extra_path).
+You can configure this with Jupyter Book like so:
+
+```yaml
+sphinx:
+  config:
+    html_extra_path: ['folder1', 'folder2']
+```
+
+When you build your book's HTML, Jupyter Book will ensure that all files and folders _inside_ the folders specified in `html_extra_path` will be copied over to your built website.
+
+For example, if you have a folder structure in your book like so:
+
+```bash
+assets
+└── data
+    └── mydataset.csv
+```
+
+and the following Jupyter Book configuration:
+
+```yaml
+sphinx:
+  config:
+    html_extra_path: ['assets']
+```
+
+Then the dataset will be accessible at `yourwebsite.com/data/mydataset.csv`.
+
+
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