Fix bug so that add_type updates forward references in new type added

[huong-li-nguyen]

Description

• Fix bug on ChildWithForwardRef.update_forward_refs having to be called
• Add unit tests to capture new test case
• Update docs

Screenshot

Without the fix, the unit tests will fail on this:

Checklist

• I have not referenced individuals, products or companies in any commits, directly or indirectly
• I have not added data or restricted code in any commits, directly or indirectly
• I have updated the docstring of any public function/class/model changed
• I have added the PR number to the change description in the changelog fragment, e.g. Enable feature XXX ([#1](https://github.com/mckinsey/vizro/pull/1)) (if applicable)
• I have added tests to cover my changes (if applicable)

Types of changes

• Docs/refactoring (non-breaking change which improves codebase)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Notice

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[antonymilne]

Thank you for finishing this off! 💯 Just some minor suggestions.

[maxschulz-COL]

Lgtm! Great fix