Ensuring all impl and test tags show in the docs

[john-science]

What is the change? Why is it being made?

There were 3 impl tags missing from requirements and 7 test tags. This was due to a combination of two things:

1. Sphinx / RST formatting errors.
2. Missing __init__.py files.

I have resolved these issues, as you can see from the PDF in the docs build on GitHub Actions:

closes #2417

SCR Information

Change Type: docs

One-Sentence Rationale: All requirements should have both test and impl tags, correct and visible in the docs.

One-line Impact on Requirements: Three requirements had Implementation tags that were not being shown in the docs but are now: R_ARMI_DEPL_TABLES, R_ARMI_DEPL_ABC, and R_ARMI_ASSEM_HEIGHT_PRES. And seven requirements had test tags that were not being rendered in the docs but are now: R_ARMI_GRID_HEX, R_ARMI_GRID_SYMMETRY, R_ARMI_GRID_SYMMETRY_LOC, R_ARMI_GRID_EQUIVALENTS, R_ARMI_GRID_HEX_TYPE, R_ARMI_GRID_GLOBAL_POS, and R_ARMI_GRID_ELEM_LOC.

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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.