documentation: clarify Overview

w/r/t "This guide aims to give the reader an overview of the structure of Rust Lightning functionality, as well as a reference for integrating the exported bindings in an object-oriented language using Swift as the example."

Some qs:
1) is this guide a walkthrough for how to create language bindings to Rust Lightning's exported C/C++ bindings, using Swift as the example language?
2) and/or, is this guide a walkthrough for how to create a RL node using this repo's Swift language bindings?

I'm still figuring out the answers to these questions, but I think the answers to those questions should be clarified in the opening paragraph. :)

Start with the Overview opening paragraph quoted in the issue and compare it with the surrounding documentation to determine whether it describes creating bindings, creating a node with Swift bindings, or both. Update that paragraph to answer the two questions explicitly, then verify the guide's wording matches its actual scope.

Written by the indexing model from the issue text.