Specify more precisely where to include repository key on init

Doc request:
```
× Github CI support requires you to specify the URL of your repository
help: Set the repository = "https://github.com/..." key in these manifests:
/home/saffron/Documents/projects/compile-typst-site/Cargo.toml
```
This should (personally) preferably mention the specific `[package]` table within the cargo toml.

Search the repository for the diagnostic text beginning “Github CI support requires you to specify the URL of your repository” and inspect where its help message is assembled. Update the guidance so it identifies the [package] table in Cargo.toml, then verify the rendered diagnostic uses the more precise wording.

Written by the indexing model from the issue text.