Add experimental support for DoD HPCMP Blueback and clean up documentation (maintainers section)

[climbfuji]

Summary

1. Add a preliminary site config for the upcoming DoD HPCMP Blueback system. This config was tested on an early-access test system for the full machine. Support is for oneAPI only until the actual machine comes online. @areinecke please review this part.
2. Clean up the maintainers section in the documentation: remove external ecflow and miniconda (both legacy) and other packages that are no longer built externally by the spack-stack team. Delete corresponding modulefile templates. JCSDA, EPIC, EMC please review this part

Note that for 2, I need help from JCSDA to fill in the missing pieces for Discover and S4.

Testing

Compiled the NEPTUNE standalone environment (template neptune-dev without py-xnrl) on the Blueback test system. This system is air-gapped. Compiled NEPTUNE using this spack-stack environment.

Applications affected

None

Systems affected

No existing system affected

Dependencies

n/a

Issue(s) addressed

n/a

Checklist

• This PR addresses one issue/problem/enhancement, or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.

[areinecke]

Looks okay with me.