docs: Fix private doc links and enable CI test (#3743)

# Objective

Fixes #3566

## Solution

- [x] Fix broken links in private docs.
- [x] Add the `--document-private-items` flag to the CI.

## Note

The following was said by @killercup in #3566:

> I don't have time to confirm this but I assume that linking to private items throws an error/warning when just running cargo doc, and --document-private-item might actually hide that warning. So to test this, you'd have to run it twice.

I tested this and this is thankfully not the case. If you are linking to a private item you will get a warning no matter if you run `cargo doc` or `cargo doc --document-private-items`.

### Example

I added `struct Test;` to `bevy_core/src/name.rs` and linked to it inside of a doc comment using ``[`Test`]``. After that I ran `cargo doc -p bevy_core --document-private-items` using `RUSTDOCFLAGS="-D warnings"` and got the following output (note the last sentence):

```rust
error: public documentation for `Name` links to private item `Test`
  --> crates/bevy_core/src/name.rs:11:82
   |
11 | /// Component used to identify an entity. Stores a hash for faster comparisons [`Test`]
   |                                                                                  ^^^^ this item is private
   |
   = note: `-D rustdoc::private-intra-doc-links` implied by `-D warnings`
   = note: this link resolves only because you passed `--document-private-items`, but will break without
```

.github/workflows/ci.yml

@@ -273,7 +273,7 @@ jobs:
       - name: Installs cargo-deadlinks
         run: cargo install --force cargo-deadlinks
       - name: Build and check doc
-        run: RUSTDOCFLAGS='-D warnings' cargo doc --workspace --all-features --no-deps
+        run: RUSTDOCFLAGS='-D warnings' cargo doc --workspace --all-features --no-deps --document-private-items
       - name: Checks dead links
         run: cargo deadlinks --dir target/doc/bevy
         continue-on-error: true

crates/bevy_ecs/src/query/iter.rs

@@ -35,7 +35,7 @@ where
     /// This does not check for mutable query correctness. To be safe, make sure mutable queries
     /// have unique access to the components they query.
     /// This does not validate that `world.id()` matches `query_state.world_id`. Calling this on a `world`
-    /// with a mismatched [`WorldId`] is unsound.
+    /// with a mismatched [`WorldId`](crate::world::WorldId) is unsound.
     pub(crate) unsafe fn new(
         world: &'w World,
         query_state: &'s QueryState<Q, F>,
@@ -175,7 +175,7 @@ where
     /// This does not check for mutable query correctness. To be safe, make sure mutable queries
     /// have unique access to the components they query.
     /// This does not validate that `world.id()` matches `query_state.world_id`. Calling this on a
-    /// `world` with a mismatched [`WorldId`] is unsound.
+    /// `world` with a mismatched [`WorldId`](crate::world::WorldId) is unsound.
     pub(crate) unsafe fn new(
         world: &'w World,
         query_state: &'s QueryState<Q, F>,

crates/bevy_ecs/src/world/mod.rs

@@ -1087,7 +1087,7 @@ impl World {
         }
     }
 
-    /// Empties queued entities and adds them to the empty [Archetype].
+    /// Empties queued entities and adds them to the empty [Archetype](crate::archetype::Archetype).
     /// This should be called before doing operations that might operate on queued entities,
     /// such as inserting a [Component].
     pub(crate) fn flush(&mut self) {