Rename migration file to `migrate_to_notebook7` (#6831)

* Use correct canonical URL while building docs (#6829)

As noted in https://github.com/readthedocs/readthedocs.org/pull/10224 with the current configuration the canonical URL is wrong with different sphinx builds. For instance the "latest" docs point to "stable". This leads to apps like Mastodon to generate previews to pages which don't exist since they rely on

    <link rel="canonical" href="https://jupyter-notebook.readthedocs.io/en/stable/notebook_7_features.html" />

This should fix that.

* Renaming migration file to `migrating_to_notebook7`

* Use the correct name

* fix ref

---------

Co-authored-by: Ashwin V. Mohanan <9155111+ashwinvis@users.noreply.github.com>

docs/source/index.md

@@ -12,7 +12,7 @@
 
 user-documentation
 configuration
-migrating
+migrate_to_notebook7
 contributor
 changelog
 ```

docs/source/notebook_7_features.md

@@ -147,4 +147,4 @@ Notebook 7 automatically switches to a more compact layout on mobile devices, ma
 This was just a quick overview of the new features in Notebook 7. For more details, you can check out the following resources:
 
 - The [JupyterLab Documentation](https://jupyterlab.readthedocs.io/en/latest/) is a great resource to learn more about JupyterLab and the extensions available. Since Notebook 7 is based on JupyterLab, many of the features and extensions available for JupyterLab are also available for Notebook 7.
-- [Migration Guide](./migrating.md) for Notebook 7, which explains how to migrate from the Classic Notebook to Notebook 7.
+- [Migration Guide](./migrate_to_notebook7.md) for Notebook 7, which explains how to migrate from the Classic Notebook to Notebook 7.