Docs: add migrate to notebook7 page (#6737)

* docs: add migrate to notebook7 page

* docs: update migrate_to_notebook7.rst

* docs: lint migrate_to_notebook7.rst

* docs: lint migrate_to_notebook7.rst

docs/source/index.rst

@@ -14,3 +14,4 @@ The Jupyter Notebook
    user-documentation
    configuration
    contributor
+   migrate_to_notebook7

docs/source/migrate_to_notebook7.rst

@@ -0,0 +1,85 @@
+Migrating to Notebook 7
+=======================
+
+Build Jupyter Notebook v7 off of JupyterLab components
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Read more details about the changes currently taking place in the
+Jupyter Ecosystem in the `JEP 79`_ and `team-compass note`_.
+
+Notebook 7 is built on top of JupyterLab components and delivers new features
+like realtime collaboration, debugger, theming.
+
+Compatibility with older versions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Any extension developed for Notebook < 7 or NbClassic will not be
+compatible with Notebook 7 and upwards.
+
+Some extensions like nbgrader have already been ported. We invite you to
+check if the extensions you are using have already been ported.
+
+Simulaneous usage of different versions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**NbClassic and Notebook 7**
+
+You can install NbClassic, Notebook 7 and JupyterLab, all three of
+which will provide different user interfaces
+on the same server. When Notebook 7 is available, the NbClassic UI, will
+be served at the ``/nbclassic/tree`` base path rather than the
+base path ``/tree`` used otherwise. If you are using Notebook 7 along
+with NbClassic, you will also have JupyterLab installed as it is a
+dependency of Notebook 7, and these front ends will be available
+through the following base paths: JupyterLab at ``/lab``, Notebook 7 at
+``/tree``, and NbClassic at ``/nbclassic/tree``.
+
+**NbClassic and Notebook 6.5.x**
+
+As NbClassic provides the static assets for Notebook 6.5.x, while
+having both installed should cause no issues, the user interface provided
+by these two packages will be the same. These UIs will be served by
+different back end servers. An NbClassic instance will be at a server with the
+``/tree`` path and opening a Notebook 6.5.x instance will open on a
+different server with the ``/tree`` path as well, presenting the same
+static assets. When starting an instance of JupyterLab you will be able
+to access the classic view of Notebook with NbClassic served on the same
+server at ``/tree``.
+
+**NbClassic and Notebook <= 6.4.x**
+
+When using NbClassic and Notebook <= 6.4.x you can expect that these UIs
+will not be only presented at different servers, meaning they will both
+be available at their respective server under ``/tree`` but they
+may also differ as potential changes to the NbClassic UI will not be
+reflected in Notebook versions <= 6.4.x. In this case as well, you would
+be able to access the classic view of Notebook with NbClassic served on
+the same server, at ``/tree``.
+
+**NbClassic and JupyterLab 3.x**
+
+When only JupyterLab 3.x is installed, then NbClassic does not have to be
+explicitly installed as JupyterLab 3.x depends on it. They will run on
+the same server, and are reachable via ``/tree`` for NbClassic and
+``/lab`` for JupyterLab.
+
+**NbClassic and JupyterLab 4.x**
+
+When only JupyterLab 4.x is installed, then NbClassic has to be installed
+explictly. They will run on the same server, and are reachable via
+``/tree`` for NbClassic, and ``/lab`` for JupyterLab.
+
+**NbClassic Independently**
+
+When you choose to install only NbClassic via ``pip install nbclassic``,
+the classic Notebook UI will be presented at the ``/tree`` path. As the
+other frontends are not installed, attempting to access the other paths
+will return errors. Note that NbClassic being a Jupyter Server extension,
+indicated Jupyter Server will be available. This provides an additional
+way to view the NbClassic frontend. You would be able to  manually
+enable the extension when running an instance of Jupyter Server,
+``> jupyter server --ServerApp.jpserver_extensions="nbclassic=True"``,
+which will provide the NbClassic frontend at ``/tree`` path when visited.
+
+.. _JEP 79: https://jupyter.org/enhancement-proposals/79-notebook-v7/notebook-v7.html
+.. _team-compass note: https://github.com/jupyter/notebook-team-compass/issues/5#issuecomment-1085254000