[bevy_ui/layout] Update internal mappings

[StrikeForceZero]

supersedes https://github.com/bevyengine/bevy/pull/12804

Objective

• Proper follow-up to #12698
  • Update UiSurface mappings to be more concise
  • Document UiSurface

Solution

• Updated mappings:
  • use EntityHashSet to track relationships between camera -> root nodes
  • adds tracking of camera_entity with Option<Entity> in RootNodeData (previously named RootNodePair)

Testing

• Did you test these changes? Only via the existing unit/e2e tests, since rebasing the original changes from https://github.com/bevyengine/bevy/pull/12804.
• Are there any parts that need more testing? Since these changes are somewhat stale, it would be worth manually confirming during runtime with various scenarios and attempts to create any undesired behavior or overlooked race conditions.
• How can other people (reviewers) test your changes?
  • [ ] Confirm behavior improves expectations or matches existing ones on the main branch
  • [ ] Adding UI Nodes
  • [ ] Removing UI Nodes
    • [ ] Despawn
    • [ ] Despawn Recursive
    • [ ] Swap Parents
  • [ ] Changing other hierarchies and order of nodes
  • [ ] Changing various style attributes (width, height, z index, and position type), resizing windows, and translating UI nodes
  • [ ] Performance improvements or regressions?