Improve readability: Format training steps as a proper numbered list in Unit 1

[delmalih]

Location: Unit 1 main page - "What Are Diffusion Models?" section
URL: https://huggingface.co/learn/diffusion-course/unit1/1

Issue:
The training steps are currently written in a single paragraph with inline numbering, which makes them harder to read and follow.

Current format:

Training the model is relatively straightforward compared to some other types of generative model. We repeatedly 1) Load in some images from the training data 2) Add noise, in different amounts. Remember, we want the model to do a good job estimating how to 'fix' (denoise) both extremely noisy images and images that are close to perfect. 3) Feed the noisy versions of the inputs into the model 4) Evaluate how well the model does at denoising these inputs 5) Use this information to update the model weights

Suggested format:

Training the model is relatively straightforward compared to some other types of generative model. We repeatedly:
1) Load in some images from the training data
2) Add noise, in different amounts. Remember, we want the model to do a good job estimating how to 'fix' (denoise) both extremely noisy images and images that are close to perfect.
3) Feed the noisy versions of the inputs into the model
4) Evaluate how well the model does at denoising these inputs
5) Use this information to update the model weights

Benefit:
This formatting improves readability by clearly separating each step, making it easier for learners to follow the training process.