doc/../rust.section.md: fix incorrect header depths

The headings for the Rust section are structured incorrectly in two ways:

1. The section "Compiling non-Rust packages that include Rust code" is totally specific to `buildRustPackage`.  It should be a child of the "Compiling Rust applications with Cargo" section.
1. The section "Setting up `nix-shell`" is totally specific to `buildRustCrate`.  It should be a child of the "Compiling Rust crates using Nix instead of Cargo" section.

- Rust
  - Compiling Rust applications with Cargo
    - ...
  - Compiling non-Rust packages that include Rust code
    - ...
  - Compiling Rust crates using Nix instead of Cargo
    - ...
  - Setting Up `nix-shell`
    - ...

- Rust
  - Compiling Rust applications with Cargo
    - ...
    - Compiling non-Rust packages that include Rust code
      - ...
  - Compiling Rust crates using Nix instead of Cargo
    - ...
    - Setting Up `nix-shell`
      - ...

doc/languages-frameworks/rust.section.md

@@ -411,13 +411,13 @@ rustPlatform.buildRustPackage rec {
 }
 ```
 
-## Compiling non-Rust packages that include Rust code {#compiling-non-rust-packages-that-include-rust-code}
+### Compiling non-Rust packages that include Rust code {#compiling-non-rust-packages-that-include-rust-code}
 
 Several non-Rust packages incorporate Rust code for performance- or
 security-sensitive parts. `rustPlatform` exposes several functions and
 hooks that can be used to integrate Cargo in non-Rust packages.
 
-### Vendoring of dependencies {#vendoring-of-dependencies}
+#### Vendoring of dependencies {#vendoring-of-dependencies}
 
 Since network access is not allowed in sandboxed builds, Rust crate
 dependencies need to be retrieved using a fetcher. `rustPlatform`
@@ -477,7 +477,7 @@ added. To find the correct hash, you can first use `lib.fakeSha256` or
 `lib.fakeHash` as a stub hash. Building `cargoDeps` will then inform
 you of the correct hash.
 
-### Hooks {#hooks}
+#### Hooks {#hooks}
 
 `rustPlatform` provides the following hooks to automate Cargo builds:
 
@@ -513,7 +513,7 @@ you of the correct hash.
 * `bindgenHook`: for crates which use `bindgen` as a build dependency, lets
  `bindgen` find `libclang` and `libclang` find the libraries in `buildInputs`.
 
-### Examples {#examples}
+#### Examples {#examples}
 
 #### Python package using `setuptools-rust` {#python-package-using-setuptools-rust}
 
@@ -863,7 +863,7 @@ features, we would write:
 
 Where `diesel.nix` is the file generated by Carnix, as explained above.
 
-## Setting Up `nix-shell` {#setting-up-nix-shell}
+### Setting Up `nix-shell` {#setting-up-nix-shell}
 
 Oftentimes you want to develop code from within `nix-shell`. Unfortunately
 `buildRustCrate` does not support common `nix-shell` operations directly