[Feature] Implement DocumentIndex Model

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/skywalking/issues?q=is%3Aissue) and found no similar feature requirement.

### Description

### Problem Statement
Currently, the index mode is tightly coupled with the measure model, which complicates the overall architecture and maintenance. To improve efficiency and scalability, we need to separate these concerns.

### Proposed Solution
I propose the creation of a new independent data model called `DocumentIndex`. This model will support the indexing of documents using an efficient indexing approach. The separation will allow for more flexible indexing and retrieval processes, making it easier to manage and adapt as our requirements evolve.

### Implementation Steps
1. Define the structure and API of the `DocumentIndex`.
2. Write integration tests to validate the indexing and document retrieval processes.
3. Update documentation to reflect the new model and its use cases.

### Use case

_No response_

### Related issues

_No response_

### Are you willing to submit a pull request to implement this on your own?

- [ ] Yes I am willing to submit a pull request on my own!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)