fix: Update Processing API reference

[emmanuel-ferdman]

Issue #, if available:

Description of changes:
Commit 3949416 moved the doc/processing.rst to doc/api/training/processing.rst. This PR adjusts sources to changes.

Testing done:

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[emmanuel-ferdman]

@Aditi2424 @chad119 @benieric @mufaddal-rohawala please review this small PR when you get the chance 🙌