docs(how-to-guides): add training docs for traffic_light_fine_detector

[lexavtanke]

Description

This PR is part of the work to provide users clear instructions how to train neural networks on their own data. See #488.

This PR adds introduction to training trafffic light detector yolox model on custom dataset and provides link to the training instruction in traffic_light_fine_detector package.

The PR should be merged after autowarefoundation/autoware_universe#5704.

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[vividf]

@kaancolak @lexavtanke
Is the PR still in progress?

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[github-actions]

Documentation URL: https://autowarefoundation.github.io/autoware-documentation/pr-489/
Modified URLs:

• https://autowarefoundation.github.io/autoware-documentation/pr-489/tutorials/training-machine-learning-models/training-models/

[xmfcx]

All links work. It's basically a link to other tutorials so I'll approve it.
Thanks for the PR @lexavtanke !