Publish a news article APIs for average, variance, and highly variable genes

[pablo-gar]

The article must follow the article editorial guidelines at:

https://github.com/chanzuckerberg/cellxgene-census/blob/main/docs/census_article_guidelines.md

The article should:

• Give a high-level overview of the features.
• Link out to the corresponding notebooks
• Explain how they work internally with a focus on mechanics rather than implementation details.

[pablo-gar]

Completed