Replace ppt generated illustrations for graphviz

[albeanth]

Description

In the documentation additions in #1033, the loose and tight coupling illustrations had Powerpoint generated illustrations as placeholders. This PR replaces those pngs with graphviz generated illustrations that are hopefully clearer and more maintainable.

#1033 (comment)

---

Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.
  • Just documentation, no tests to be added.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any bug fixes or new features.
  • Not changing text in documentation, just images.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.

[albeanth]

Here's what the new images look like when the docs are built.

[opotowsky]

nice!

[john-science]

I love it! @albeanth

Are you SURE we don't need to add any dependencies to the repo to make this work?

If so, GREAT, I'm thrilled.

[albeanth]

I love it! @albeanth

Are you SURE we don't need to add any dependencies to the repo to make this work?

If so, GREAT, I'm thrilled.

Yep! No additional dependencies.

The one thing that came up when I was making this. I didn't see any reference in the docs about having to run https://github.com/terrapower/armi/blob/main/doc/requirements-docs.txt. If that is not run, then the docs do build locally, but they're all messed up.