tidy: Add support for .txx file extension in Doxyfile

[KSkwarczynski]

Pull request description

Here Luke added .txx here: #824

Changes or fixes

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Memory and CPU Usage Summary

### Memory and CPU Usage Summary

| Metric  |     Mean    |      Max    |
|---------|-------------|-------------|
| **RAM** |   354.86 MB |   358.69 MB |
| **CPU** |    97.28 %  |   108.25 %  |

Memory Usage Plot

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