[material-ui] Refine and unify custom template themes

[zanivan]

• I have followed (at least) the PR section of the contributing guide.

This PR is to unify and standardize custom themes and its structure across all the templates.

The only difference is that in the Dashboard template, we'll have some extra files for X's components such as DataGrid, DatePicker, TreeView and Charts.

For the structure, I follow the same grouping as Material UI's docs, grouping custom components style by Inputs, DataDisplay, Feedback, Surfaces, Navigation, Layout, Utils:

├── template
│   ├── components
│   │   ├── AppAppBar.tsx
│   │   ├── Chart.tsx
│   │   ├── Drawer.tsx
│   │   └── ...
│   ├── theme
│   │   ├── customizations
│   │   │   ├── index.tsx
│   │   │   ├── inputs.tsx
│   │   │   ├── dataDisplay.tsx
│   │   │   ├── feedback.tsx
│   │   │   ├── surfaces.tsx
│   │   │   └── ...
│   │   ├── getTemplateTheme.tsx
│   │   └── themePrimitives.tsx
│   ├── Template.tsx
│   └── README.md

👉 https://deploy-preview-43220--material-ui.netlify.app/material-ui/getting-started/templates/

[mui-bot]

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[aarongarciah]

@zanivan I rebased on top of next to solve a CI issue.

[aarongarciah]

Great job!

Note: I haven't reviewed every single file, but overall looks good.