Add a large deprecation notice to the front-end extensions documentation

[choldgraf]

Context

Currently, the plan is for the next major version of the Jupyter Notebook to use JupyterLab components. However, many people are still using and developing on the v6 notebook setup. This is particularly challenging for people building extensions that depend on APIs that will soon be removed. For example, I recently found out that a large university started building a custom grading UI on top of the v6 notebook UI, because they didn't realize it was going to be switched to JupyterLab.

Proposal

I suggest that we add a large ‼️ DEPRECATION to any of the current documentation pages that discuss extension APIs that will be deprecated. For example, the front-end interface page. We can then include a link to the Jupyter Notebook v7 plan and help clarify confusion before people are surprised.

[jtpio]

Thanks @choldgraf that sounds like a good idea 👍

For reference the latest docs have already been updated with very basics instructions for making extensions for lab 4 / nb 7 (but can definitely be iterated on): https://jupyter-notebook.readthedocs.io/en/latest/extending/frontend_extensions.html

[jtpio]

Some improvements to the docs to mention the migration to Notebook 7 landed in #6737.

But we still need to rework the front-end documentation page.

[jtpio]

Maybe this change would need to target 6.5.x to the warning is visible on the Classic Notebook v6 documentation. For example: https://jupyter-notebook.readthedocs.io/en/v6.5.4/

[RRosio]

I started a PR to address this in 6.5.x by adding a warning directive. Would adding this to the different pages with deprecated APIs be similar enough to the proposed change?

• Maybe instead of a general link to Notebook 7 docs, the link can be to the Extending the Notebook section of the docs?
• Also, are there some server docs that might still benefit from the warning even if the API is not necessarily entirely different?

** Other pages where I am thinking a warning would be included:
- Customize Keymaps>> Point to Keyboard Shortcuts
- Bundler extensions >> Point to Many Jupyterlab-extensions (?)

[jtpio]

• Also, are there some server docs that might still benefit from the warning even if the API is not necessarily entirely different?

For the server docs maybe a small message and pointing to the Jupyter Server docs would be enough?