Reorganize machine-independent vs. Polaris-specific example PBS submission scripts

[felker]

Copied from Polaris channel on CELS Slack. Related #218

The Polaris “Running Jobs” page is a bit messed up right now: https://docs.alcf.anl.gov/polaris/running-jobs/
@weallcock extracted/moved most of the Polaris example PBS scripts from the Polaris sidebar/subdirectory in #122 which he started in November that I merged in February; the idea being that many of the PBS examples will have machine-independent lessons for when we make Sunspot/Aurora public?

But it is in a weird state right now, since the generic https://docs.alcf.anl.gov/running-jobs/example-job-scripts/ basically reads like a Polaris page, with many CPU core-count and GPU model-specific lessons.

@zippylab had some concerns to that effect in #122 (comment) and made a small patch last month to supplement the Polaris page in #199 but I dont recall if we ever actually discussed/addressed the original concerns.

So more possible changes to https://docs.alcf.anl.gov/polaris/running-jobs/ for now:

• add more links to the “generic” running jobs/example job scripts page
• make it clear that the first example doesnt assume anything about the GPU usage; i.e. it could be a pure CPU application
• Running MPI+OpenMP Applications
• Use pymdownx.snippets to avoid duplicating source documentation for machine-independent PBS info. See Add comparison pymdownx.snippets to codeinclude #177

Not sure how to improve https://docs.alcf.anl.gov/running-jobs/example-job-scripts/

• @zippylab suggests the Example Job Scripts section should just be moved under Polaris Running Jobs. Maybe rename it Polaris Example Job Scripts. Even CPU thread mappings and CPU-GPU connectivity details are machine specific.
• @cjknight: cpu and ensemble stuff might be ok in generic pbs page and gpu bits could be put in machine-specific page (like Tim suggesting?)
• Start adding non-restricted info about Sunspot job scheduler to the page, so it would be good to have another example just to see how we will structure and write a machine-independent, generic guide
• How many machines do we expect to have that will be running PBS and not Cobalt in the near future?