coral2: deprecate docs

Problem: the `flux-coral2` repo provides its own documentation that
is more up-to-date than what is provided here.

Deprecate the documentation here, with links to the new docs.

Author: jameshcorbett

tutorials/lab/coral2.rst

@@ -12,7 +12,3 @@ provided there are for collaborating systems.
 
    LLNL Introduction to Flux <https://hpc-tutorials.llnl.gov/flux/>
    coral
-   coral2
-   rabbit
-   rabbit_config
-