Provide instructions on how to update `api.json` while using a custom Godot build

In "[Using custom builds of Godot](https://godot-rust.github.io/book/gdnative/advanced/custom-godot.html#using-custom-builds-of-godot)" section, the book should provide instructions on how to update the automatically generated `api.json` file after upgrading the custom build.

I solved this problem by going to `~/.cargo/git/checkouts/godot-rust-1005fa777d499fc2/` and executing
```sh
find -name "api.json" -delete
```
After that, I removed the `target` folder in my Rust project to trigger a clean build from scratch. Not sure whether this is the best method. Please let me know if there's a better alternative.

Start with the “Using custom builds of Godot” section at the linked custom-godot.html location and verify how the generated api.json is refreshed after a custom build upgrade. Compare the reported deletion of api.json and the Rust project's target folder with the supported workflow. Done means the section documents reliable, reproducible update instructions.

Written by the indexing model from the issue text.