`check-missing-*-in-docs` job does not install Rust

In `ci.yml`, both `check-missing-examples-in-docs` and `check-missing-features-in-docs` do not install Rust before running it. This means that they are instead using the [implicitly installed Rust tools](https://github.com/actions/runner-images/blob/7861e423d5d645b70e34148c5c35f22ebf1c4e2c/images/ubuntu/Ubuntu2204-Readme.md#rust-tools) that come with the `ubuntu-latest` image.

This is an issue because the image versions of Rust may not update in lockstep with the rest of the Bevy project. Our current MSRV policy is "latest stable." If the image installation is behind, it may cause the jobs to spuriously fail.

To fix this, [`dtolnay/rust-toolchain@stable`](https://github.com/dtolnay/rust-toolchain) should used. See other jobs [such as `build`](https://github.com/bevyengine/bevy/blob/1df41b79dd5cec243046b2b5acce496ae3ec1dec/.github/workflows/ci.yml#L36) for reference.

https://github.com/bevyengine/bevy/blob/1df41b79dd5cec243046b2b5acce496ae3ec1dec/.github/workflows/ci.yml#L308-L317

https://github.com/bevyengine/bevy/blob/1df41b79dd5cec243046b2b5acce496ae3ec1dec/.github/workflows/ci.yml#L341-L349