[Rust] Document persistent streams

[elenagaljak-db]

What changes are proposed in this pull request?

This PR adds the user-facing documentation for the feature-gated persistent-stream API introduced by the preceding stack.

It adds a runnable JSON example that:

• Creates a new durable stream when ZEROBUS_PERSISTENT_STREAM_ID is unset.
• Prints the server-assigned stream_id so applications can persist it externally.
• Resumes an existing stream when ZEROBUS_PERSISTENT_STREAM_ID is set.
• Queues records in a loop and calls flush() once afterward, preserving the SDK ingestion pipeline.
• Explains that close() ends the current connection without retiring the durable stream.

The Rust examples index and JSON README document how to run both the create and resume paths. This PR also adds the documentation entry to rust/NEXT_CHANGELOG.md.

Stack

1. [Rust] Add persistent stream proto #761 — protobuf contract
2. [Rust] Add persistent gRPC transport #762 — gRPC transport and recovery engine
3. [Rust] Expose persistent stream API #763 — public Rust API
4. [Rust] Test persistent stream resume #764 — stateful mock and integration tests
5. Documentation and runnable example

How is this tested?

• cargo check -p rust-examples-json --example json_persistent
• cargo test -p tests --test persistent_stream_tests — 2 tests passed
• cargo fmt --all -- --check
• git diff --check