docs(arc-ui): update readme

[vaibhavbhalla2505]

This pull request updates the documentation for the Angular Search Library to improve clarity, update usage instructions for newer Angular versions, and enhance theming and styling guidance. The most important changes include clarifying Angular version compatibility, updating usage examples for standalone components, and providing detailed theming options using CSS variables.

Documentation improvements and Angular compatibility:

• Replaced the deprecation notice with a detailed compatibility table mapping Angular versions to the correct @sourceloop/search-client versions, making it easier for users to select the appropriate package version.
• Updated usage instructions to reflect that SearchComponent is now a standalone component, removing the need for NgModule imports and providing an updated example for integration.

API and configuration documentation:

• Removed the documentation for the deprecated dropDownButtonIconClass property and the "show only result overlay" configuration, streamlining the list of supported and recommended configuration options. [1] [2]
• Standardized code examples and configuration object values to use single quotes for consistency. [1] [2] [3] [4]

Styling and theming guidance:

• Added a new section detailing required global styles, introduced available CSS variables for theming, and provided example usage for customizing the search component's appearance to match an application's design system.