[Feature] Paragraph about monitoring HPC

**Is your feature request related to a problem? Please describe.**
Follow up to https://github.com/brainglobe/brainglobe.github.io/issues/457

**Describe the solution you'd like**
The HowTo guide should include high-level instructions and links around how to monitor the HPC job, including
* where to look for intermediate output files at different stages
* what slurm jobs we expect to spawn