fix(theme): add mui v4 and v5 grid examples

[christoph-jerolimov]

Hey, I just made a Pull Request!

This PR adds

1. some new Grid examples to test plugins to see and understand the difference between Backstage and RHDH. But there is none; both look except different colors and borders really similar!
2. add workaround to load mui v5 theme correctly when package test plugins

Backstage defines a default spacing for all Grids, see:

https://github.com/backstage/backstage/blob/3d2e95f71c2be9a63530697cf01e482e4f50f98d/packages/theme/src/v5/defaultComponentThemes.ts#L67-L71

The RHDH theme doesn't change the padding or default spacing. But in MUI 5 it's required to add additional padding via spacing to the Grid container or to the content (in the Grid item).

The UI will looks unaligned when a padding is added to the Grid item.

MUI v4 and v5 looks similar if no padding is added to the Grid item

But it's not okay to add padding to Grid item in MUI v5:

MUI v4

MUI v5

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or Updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)

[rhdh-gh-app]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@red-hat-developer-hub/backstage-plugin-bc-test	workspaces/theme/plugins/bc-test	none	v0.1.0
@red-hat-developer-hub/backstage-plugin-mui4-test	workspaces/theme/plugins/mui4-test	none	v0.1.0
@red-hat-developer-hub/backstage-plugin-mui5-test	workspaces/theme/plugins/mui5-test	none	v0.1.0