Moving dochelpers out of the ARMI namespace

[john-science]

What is the change?

This PR moves dochelpers out of the ARMI namespace.

In particular, it splits the file armi/utils/dochelpers.py into two parts and puts them in:

• doc/conf.py - The classes ExecDirective and PyReverse were only ever used in our conf.py anyway.
• doc/.static/dochelpers.py - There were some really simple helper functions here imported by the docs.

Why is the change being made?

The purpose of this PR is two-fold:

1. This code should never have been part of the ARMI namespace; it has nothing to do with nuclear modeling.
2. To remove docutils as a primary dependency of 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 commit message(s) follow 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]

This PR is dependent upon this other PR being merged first: #1660

[opotowsky]

Looks good to me.

[tp-pbritt-nuc]

@john-science This change broke Pylot docs. Could you take a look when you have a chance?