feat(examples): add integration examples for Actix, Axum, and Rocket

[Maki-Grz]

This PR adds documentation examples demonstrating how to integrate the Qdrant client with the most popular Rust web frameworks: Actix-web, Axum, and Rocket.

As gRPC response types (from prost) do not implement serde::Serialize by default, these examples demonstrate the recommended pattern: sharing the client via application state and using serde_json::json! as a bridge to return serializable JSON to web clients.

Changes

• Updated Cargo.toml: Added frameworks to [dev-dependencies] and declared new [[example]] sections.
• examples/actix_web.rs: Demonstrates state sharing via web::Data.
• examples/axum.rs: Demonstrates state sharing via extract::State.
• examples/rocket.rs: Demonstrates managed state via rocket::State.

How to test

1. Run a Qdrant instance: docker run -p 6333:6333 -p 6334:6334 qdrant/qdrant
2. Run any example (features are handled automatically via required-features):

   cargo run --example axum

Query the endpoint: curl http://localhost:3000/collections

All Submissions:

• Contributions should target the dev branch.
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests?

New Feature Submissions:

• Does your submission pass tests? (Verified via cargo check --examples)
• Have you formatted your code locally using cargo +nightly fmt --all?
• Have you checked your code using cargo clippy --all --all-features?

Changes to Core Features:

• Have you added an explanation of what your changes do and why?
• Have you written new tests for your core changes?
• Have you successfully ran tests with your changes locally?
  (N/A: This PR only adds documentation examples)