[train][docs] add Ray Train logo (#39572) (#39576)

Signed-off-by: Matthew Deng <[email protected]>

Author: matthewdeng

doc/source/train/images/logo.png

@@ -3,6 +3,14 @@
 Ray Train: Scalable Model Training
 ==================================
 
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+.. figure:: images/logo.png
+   :align: center
+   :width: 50%
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 Ray Train is a scalable machine learning library for distributed training and fine-tuning.
 
 Ray Train allows you to scale model training code from a single machine to a cluster of machines in the cloud, and abstracts away the complexities of distributed computing.