feat: Add demo videos and relocate banner image

This commit introduces a new "Videos" section in the `README.md` file, showcasing demonstrations of various UI components. It also relocates the `banner.png` image to a new `assets` directory.

The following changes were made:
- **README.md**:
    - Updated the path for `banner.png` from the root directory to `assets/banner.png`.
    - Added a new "Videos" section with a table displaying four MP4 videos:
        - `assets/button-group.mp4`
        - `assets/buttons.mp4`
        - `assets/floating toolbar.mp4`
        - `assets/progress indicator.mp4`
- **File System**:
    - The image `banner.png` has been moved from the project root to `assets/banner.png`.
    - The following new video files have been added to the `assets` directory:
        - `assets/button-group.mp4`
        - `assets/buttons.mp4`
        - `assets/floating toolbar.mp4`
        - `assets/progress indicator.mp4`