Update QuadSceneIndex Example

[raph080]

Description of Change(s)

The custom SceneIndex example from "Hydra 2.0 Getting Started Guide" (QuadSceneIndex) seems out of date. I suggest some update/fixes in this PR.

Fixes Issue(s)

• Replace the BuildRetained calls (currently deprecated) in favour of Builder().Build().
• Fix the signature of the GetChildPrimPaths method (missing const).
• Fix a typo for one of the return instruction of GetChildPrimPaths (misplaced semicolon).

Suggestion(s)

• Remove the Populate method from the QuadSceneIndex since it seems unnecessary for this simple example.
• Create local using in order to ease the readability.
  • e.g using _BoolDataSource = HdRetainedTypedSampledDataSource<bool>; gives _BoolDataSource::New(false).

• [x] I have verified that all unit tests pass with the proposed changes

• [x] I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9793

✅ Signed CLA on file

[jesschimein]

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