Add link to NCCL analysis paper to NCCL page (#259)

Author: msimberg

.github/actions/spelling/allow.txt

@@ -140,6 +140,7 @@ customised
 cyberduck
 dbcsr
 dcomex
+demystifying
 dftb
 dftd
 diagonalisation

docs/software/communication/nccl.md

@@ -20,6 +20,8 @@ While the container engine sets these automatically when using the NCCL hook, th
 --8<-- "docs/software/communication/nccl_env_vars"
 ```
 
+[_Demystifying NCCL: An In-depth Analysis of GPU Communication Protocols and Algorithms_](https://arxiv.org/abs/2507.04786v2) contains detailed information about NCCL algorithms and protocols, which can be helpful for deciding if your application could benefit from an alternative configuration.
+
 !!! warning "Using NCCL with uenvs"
     The environment variables listed above are not set automatically when using uenvs.