[DX] ☂️ Update the documentation on README.md and Example Apps

[salihgueler]

What feature would you like to see?

The documentation on the package is outdated at few places and feels like lacking a "follow to learn approach". Here are the feedbacks that I have for the docs (more will be added) and reference PRs to fix it.

• [x] Usage is misplaced at the bottom. It needs to be on top to help with getting started (https://github.com/woltapp/wolt_modal_sheet/pull/285)
• [x] Library examples part of sample apps. We need a basic example without any external dependency. (https://github.com/woltapp/wolt_modal_sheet/pull/285)
• [x] Some code samples are outdated in README.md. (https://github.com/woltapp/wolt_modal_sheet/pull/285)
• [ ] Features are explained but they are lacking code snippets to support them.
• [ ] Sample apps are complicated. Add an example app to the sample apps as well.
• [ ] Sample Apps missing README.md in them.
• [ ] Sample Apps have inconsistent padding and shadow effects.