add: controlnet entry to training section in the docs. (#2677)
* add: controlnet entry to training section in the docs.
* formatting.
* Apply suggestions from code review
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
* wrap in a tip block.
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Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>