Addition of generic workflows from `ACCESS-NRI/access-om2-configs`

[CodeGat]

This PR moves the deleted workflows from https://github.com/ACCESS-NRI/access-om2-configs/pull/117/files into this repo, references the newly-moved workflows, fixes some bugs, and adds a fresh coat of paint.

In this PR:

• Addition of pr-* and schedule-* workflows from https://github.com/ACCESS-NRI/access-om2-configs/pull/117 in https://github.com/ACCESS-NRI/model-config-tests/pull/24/commits/13cd82771a4a68e6f93397c9be4725a66ea4cac2
• Addition of parse-ci-config action, also in https://github.com/ACCESS-NRI/model-config-tests/pull/24/commits/13cd82771a4a68e6f93397c9be4725a66ea4cac2
• Generification of the above workflows - for those changes see https://github.com/ACCESS-NRI/model-config-tests/pull/24/files/13cd82771a4a68e6f93397c9be4725a66ea4cac2..c9d618448dc5ac7f3aca131f81c6cd416bce1d7d, but they include:
  • Replacing old run: gh pr comment ... commands with uses: access-nri/actions/.github/actions/pr-comment@main actions
  • Removing unneeded checkouts and env: sections
  • Renaming of generate-initial-checksums.yml -> generate-checksums.yml in https://github.com/ACCESS-NRI/model-config-tests/pull/24/commits/b87ed06167e602a6fca607acad659f1e09222a20
  • Renaming of config-pr-checks.yml to test-repro.yml in https://github.com/ACCESS-NRI/model-config-tests/pull/24/commits/9b5645b32d2e49a833c71d5b128ab137aae36db6 and removing the model-name input
  • Adding validation to the config/ci.json, just in case, in https://github.com/ACCESS-NRI/model-config-tests/pull/24/commits/1d6a1721cd0d7b47df4c37264a11ef92548e0f40
  • And fixing of a caching bug from https://github.com/ACCESS-NRI/model-config-tests/pull/22 in https://github.com/ACCESS-NRI/model-config-tests/pull/24/commits/76157f8af4b03be53ffeed1b171c4d025428fcf4
• README.md: new reusable workflows
• CI.yml: Ignore running checks on PRs that modify CI/CD and some other supplementary files

References #23

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 14.44%. Comparing base (9d3c1b1) to head (c9d6184). Report is 1 commits behind head on main.

:exclamation: Current head c9d6184 differs from pull request most recent head 59da225

Please upload reports for the commit 59da225 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #24 +/- ## ======================================= Coverage 14.44% 14.44% ======================================= Files 11 11 Lines 547 547 ======================================= Hits 79 79 Misses 468 468 ```

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[CodeGat]

@jo-basevi feel free to commit to this one if it's related :)