Making the demo pages better looking

[czp13]

Description

Based on Jonte's ideas: more padding, flex display, and flex column changes.

Before (zoomed out):

After (zoomed out):

Fixes # (issue)

• https://github.com/vaadin/form-filler-addon/issues/120

Type of change

• [x] Bugfix
• [ ] Feature

Checklist

• [x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• [x] I have added a description following the guideline.
• [x] The issue is created in the corresponding repository and I have referenced it.
• [x] I have added tests to ensure my change is effective and works as intended.
• [x] New and existing tests are passing locally with my change.
• [x] I have performed self-review and corrected misspellings.

Additional for Feature type of change

• [ ] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.