[DOC] - Add banner pointing to new docs at `nebari.dev`

[pavithraes]

We need to add a banner to the old docs at qhub.dev that mentions:

• the docs aren't maintained and will soon not be available
• the current docs are at nebari.dev

To add a banner:

• pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/announcements.html#provide-local-html-in-your-theme
• https://jupyterbook.org/en/stable/web/announcements.html?highlight=announcements#announcement-banners

[pavithraes]

We have jupyter-book as a requirement for the docs (we rely on jupyter-book's list of dependencies), which in turn needs sphinx<5 -- this means we can't use the newer pydata-sphinx-theme versions which have built-in announcement banners.

Solution: Create a custom "template" to display the announcement.

Note that our pydata-sphinx-theme version is 0.9.0: https://pydata-sphinx-theme.readthedocs.io/en/v0.9.0/index.html