Create design docs for NeuXtalViz components

Summary of Changes

This MR proposes the following changes:

1. Refactoring this library into an MVVM structure.
2. Adding a new theme to make Trame applications more compact.
3. Adding Qt-style layout classes (Grid, HBox, and VBox for now since those are all that NeuXtalViz uses).
4. Adding Mantid and validation models.
5. Adding views for 2D and 3D plotting of Mantid workspaces.

Checklist

• The MR has a clear and concise title
• Code is self-documented and follows style guidelines.
• Automated tests are written and pass successfully.
• Regression tests (e.g. manually triggered system tests, manual GUI/tool tests, ...) are performed to make sure the MR does not break anything (when applicable)
• Readme file is present and up-to-date.

Documentation Updates

https://nova-application-development.readthedocs.io/en/latest/index.html has been updated with the contents of this branch.

Additional Notes

Closes #28 (closed)