Fixing material doc header levels

[john-science]

What is the change? Why is it being made?

The goal of this PR was to fix the RST header levels in the docs for the material input files. This was just a simple RST mistake.

HOWEVER, it looks like when we first made the original docs PR for this, we got a little wild with making in-line changes to the docs in the GH PR page... and that completely wrecked the docs in the RST format. They were Hideous and hard to read; no rhyme or reason to the line length at all. Anyway, I fixed that, so that's going to make this PR a mess to read. And I'm sorry about that.

close #2542

SCR Information

Change Type: docs

One-Sentence Rationale: Our users navigate the docs using the headers, so they should be ordered correctly.

One-line Impact on Requirements: NA

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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 documentation is still up-to-date in the doc folder.
• The code style follows good practices.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

@opotowsky The PDF looks like it works fine:

[opotowsky]