[docs] Update installation instructions for Python 3.13

@Innixma

LibOMP does not require using custom formula on MacOS anymore (see [timeseries] Fix platform tests for timeseries #5423). LibOMP can be installed with just brew install libomp instead of the old path with wget @Innixma @prateekdesai04
Mention that Python 3.13 remains experimental since Ray is marked "beta" on py313 on MacOS/Linux, and offers no supports on Windows. @Innixma @prateekdesai04
Some AutoMM dependencies like MMCV are not available on Python 3.13 @FANGAreNotGnu
```
!mim install "mmcv==2.1.0"
!pip install "mmdet==3.2.0"
```

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