Adding an SCR section to the docs

[john-science]

What is the change?

Here I add a new process to ARMI. Instead of just a random "release notes" section, every ARMI PR will come with an SCR.

NOTE 1: This PR will be the first PR merged after the release of ARMI 0.5.1.

(NOTE 2: To reduce merge conflicts, I have not moved over the 0.5.rst release notes yet. I will do that before I merge this.)

Why is the change being made?

This is part of the on-going quality work in ARMI.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.
• The code style follows good practices.
• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

Here is a little proof this works in HTML:

And you can see proof this works in PDF here: https://github.com/terrapower/armi/actions/runs/13725317705?pr=2090