GAC costing documentation into template dedicated file

[hunterbarber]

Fixes/Resolves:

GAC costing documentation for #1360
Mostly lateral changes from unit model documentation to costing model documentation files with minor wording changes

Summary/Motivation:

Cost documentation for all unit models

Changes proposed in this PR:

• Move costing documentation from GAC unit model doc to new cost model documentation generated from the template

Legal Acknowledgement

By contributing to this software project, I agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.84%. Comparing base (38735ec) to head (b3aa409).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1375   +/-   ##
=======================================
  Coverage   93.84%   93.84%           
=======================================
  Files         339      339           
  Lines       35890    35890           
=======================================
  Hits        33682    33682           
  Misses       2208     2208           

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[MarcusHolly]

LGTM, my only suggestion would be to replace instances of USD\_2020 with what I added in the comment below.

[savannahsakhai]

LGTM.

[lbianchi-lbl]

CI failures are unrelated and most likely due to #1158. I've restarted the failed job which should result in this PR getting automerged once it passes.