docs: HPC documentation

[germa89]

As the title.

Before starting to work on #2961 I thought it will be better to summarise how we can currently run vanilla PyMAPDL commands in an HPC cluster.

This PR does not account for #2754, so it should be merged before that one.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.14%. Comparing base (5442a4e) to head (755f7fa).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2966      +/-   ##
==========================================
- Coverage   86.63%   84.14%   -2.50%     
==========================================
  Files          52       52              
  Lines        9498     9498              
==========================================
- Hits         8229     7992     -237     
- Misses       1269     1506     +237     

[germa89]

Pinging @mcMunich @pmaroneh @mikerife for visibility, awareness, and feedback.

Pinging @PipKat for documentation review.

You might want to use the Rich diff button in the Files changed tab:

[PipKat]

Lots of editorial comments for you!