Correct and expand the WebSockets documentation

[Vectornaut]

Summary

The branch to be merged corrects some errors that prevented the WebSockets sample code from compiling.

Changes

Corrections

The errors corrected include the ones described in issue #77, which affect the code in the "Writing a WebSocket controller" section.

They also include an additional error in the "Starting a WebSocket server" section; the code from this section is needed to build a runnable program.

Expansion

The branch to be merged also expands the testing instructions to confirm that the echo server is working as intended.

Validation

I've checked my changes using the following process.

1. Start a new crate with the following Cargo.toml.

   [package]
   name = "websocket-docs-test"
   version = "0.1.0"
   edition = "2024"
   
   [dependencies]
   rwf = "0.2.1"

2. Copy the code from the corrected documentation sections into src/main.rs.

3. Start the server with cargo run and then follow the expanded testing instructions from the documentation.

[Vectornaut]

Thanks for the review, @levkk! I've marked this as a draft while waiting for pull request #81 to be merged. Once that happens, this branch should be able to pass its continuous integration checks (maybe after rebasing onto #81), and I'll open it again.

[Vectornaut]

Rebased onto #81; continuous integration checks now pass! Merging.