Doc: NERSC Update (#463)

* Doc: NERSC Update

- Update Perlmutter
- Remove Cori
- Link WarpX docs for more machines

* Cyclotron: No Plot Script Yet

Fix a warning

* Doc: Remove one more Cori Mention

* Docs: Remove Gitter, Use GH Discussions

docs/source/index.rst

@@ -17,7 +17,7 @@ This is the next generation of the `IMPACT-Z <https://github.com/impact-lbl/IMPA
 Contact us
 ^^^^^^^^^^
 
-If you are starting using ImpactX, or if you have a user question, please pop in our `Gitter chat room <https://gitter.im/ECP-WarpX/community>`__ and get in touch with the community.
+If you are starting using ImpactX, or if you have a user question, please pop in our `discussions page <https://github.com/ECP-WarpX/impactx/discussions>`__ and get in touch with the community.
 
 The `ImpactX GitHub repo <https://github.com/ECP-WarpX/impactx>`__ is the main communication platform.
 Have a look at the action icons on the top right of the web page: feel free to watch the repo if you want to receive updates, or to star the repo to support the project.

docs/source/install/hpc.rst

@@ -24,17 +24,11 @@ HPC Systems
 .. toctree::
    :maxdepth: 1
 
-   hpc/cori
    hpc/perlmutter
-..   hpc/summit
-..   hpc/spock
-..   hpc/juwels
-..   hpc/lassen
-..   hpc/quartz
-..   hpc/lawrencium
-..   hpc/ookami
 
 .. tip::
 
    Your HPC system is not in the list?
-   `Open an issue <https://github.com/ECP-WarpX/impactx/issues>`__ and together we can document it!
+   Our instructions are nearly identical to `installing WarpX, documented here <https://warpx.readthedocs.io/en/latest/install/hpc.html#hpc-machines>`__.
+
+   Also, please do not hesitate to `open an issue <https://github.com/ECP-WarpX/impactx/issues>`__ and together we can document how to run on your preferred system!