Provide examples of `processing` extension

[fmigneault]

🚀 Feature Request

The section https://github.com/stac-extensions/mlm/blob/main/README.md#model-band-or-variable-object provides references to:

• Add more example expression objects processing#31
• add more processing:expression examples for python, docker and generic URI processing#33

In order to guide toward using processing extension to describe the script that is used to invoked the model (training or inference).

More concrete examples should be provided in best practices.

🔉 Motivation

Make it clear about the best practices and recommended strategies for alignment between users.

📡 Alternatives

• application extension could be used either conjointly or as replacement
• Consider application extension for processing script reference #117