Adding JupyterBook to the footer (jupyter#823)

Co-authored-by: Chris Holdgraf <choldgraf@gmail.com>
Co-authored-by: Michał Krassowski <5832902+krassowski@users.noreply.github.com>

Author: 3 people

_layouts/default.html

@@ -23,6 +23,8 @@
 projects:
   - title: Binder
     url: /binder
+  - title: Jupyter Book
+    url: https://jupyterbook.org/
   - title: JupyterHub
     url: /hub
   - title: JupyterLab