Document private items in dev-docs (bevyengine#14769)

# Objective

- The [developer docs](https://dev-docs.bevyengine.org) are primarily
used by developers, who often deal with internal and private functions.
- @mweatherley suggested [on
Discord](https://discord.com/channels/691052431525675048/743559241461399582/1273658470134186112)
that CI passes `--document-private-items` to the dev-docs, so
contributors can easily browse these internal details.

## Solution

- Add `--document-private-items` to the CI job that builds the dev-docs.

## Testing

- It should run in CI and generate documentation for private code.

## Drawbacks

- Users that track the main branch may use the dev-docs instead of
<https://docs.rs/bevy>, which may now show a lot of unwanted internal
details.
- Searching may be slower / bloated with internal details.

.github/workflows/docs.yml

@@ -67,6 +67,7 @@ jobs:
             --all-features \
             --workspace \
             --no-deps \
+            --document-private-items \
             --exclude ci \
             --exclude errors \
             --exclude bevy_mobile_example \