[FEATURE] Improve documentation with Flint overview and doctest examples

[dai-chen]

Is your feature request related to a problem?

The current documentation lacks clear, executable examples, making it challenging for users to understand and test the repo's features effectively. This can cause confusion, especially for new users exploring the repository for the first time.

What solution would you like?

• Clarify the purpose of the Flint library: Provide a clear explanation of what Flint is to avoid confusion. If Flint is meant to be internal, consider removing its references from the README.
• Provide usage instructions with Spark: Include step-by-step guidance on how to use the Flint library independently with Spark for specific use cases.
• Add executable examples: Incorporate clear, runnable examples in the documentation using doctest or a similar framework to showcase common use cases. These examples help users quickly validate and understand the library's capabilities.
• Include architecture and design documentation: Add architecture and design details by referencing key PRs that contain important design explanations. This will provide users with a deeper understanding of the system's design and how the components interact.

What alternatives have you considered?

N/A

Do you have any additional context?

Issue related: #1008