[Doc][Improvement] Re-organize the ReadMe to make it more friendly to beginners

**Is your feature request related to a problem? Please describe.**
The current ReadMe of GraphAr mainly focuses on the file format design. While the format design is the most important part of GraphAr, which is a standard file format for graphs, we can refine and improve the organization of the ReadMe to make it more user-friendly for various audiences. This includes those who want to learn about the file format and those who want to quickly get started with using the libraries to import/export data.

**Describe the solution you'd like**
- Highlight the key features and benefits of GraphAr, as well as the various scenarios in which it can be used.
- Provide more detailed information on how to get started with the libraries.

Review the current README and its existing sections on GraphAr's file format. Outline a friendlier structure that highlights features, benefits, use cases, and how to get started with the import/export libraries. Done means the README serves both readers learning the format and users seeking a quick library introduction.

Written by the indexing model from the issue text.