Add a count of how many settings are in ARMI

[john-science]

What is the change? Why is it being made?

The documentation currently contains a "Settings Report". This PR is a very small change to that report; right at the start it adds a simple count of how many Settings ARMI defines.

This is not a huge change, but I found myself wanting to know the answer. And I would also like to watch and see how this number changes over time.

SCR Information

Change Type: docs

One-Sentence Rationale: For long-term tracking, it would be nice to see how many Settings are defined in ARMI.

One-line Impact on Requirements: NA

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Checklist

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

[john-science]

WIP: My first attempt failed. The entire sentence is missing from the docs.