RISE is obsolete and this should be made clear in the doc - redirect to jupyterlab-rise

[parmentelat]

as outlined in mwouts/jupytext#1126
the current documentation does not reflect clearly enough that

• RISE only supports nb classic, which is obsoleted
• there is a jupyterlab alternative, and how to install and use it

[parmentelat]

for the record I have disabled the stable build at the readthedocs project,
because it pointed at an older version that I can no longer modify
use either master or latest like e.g.
https://rise.readthedocs.io/en/latest/index.html