Update best practices for `example` reference with container or language

[fmigneault]

🚀 Feature Request

The example exntesion allows for example:container and example:language fields (https://github.com/stac-extensions/example-links#fields). However, those are only allowed in links (not in assets).

Given stac-extensions/example-links#4 proposal, if example allowed for the fields to be added to an Asset, they could be used to provide details about a reference code/container along the model (eg: a training pipeline script).

🔉 Motivation

Improve the documentation about using the model.

📡 Alternatives

• processing:expression can be used, with the extended URI/media-type (stac-extensions/processing#31). The https://github.com/stac-extensions/mlm/blob/main/best-practices.md#processing-extension section should be updated accordingly.

• Still use the example in links as supplementary material.
  More details and examples should be added to https://github.com/stac-extensions/mlm/blob/main/best-practices.md#example-extension

📎 Additional context

• stac-extensions/example-links#4