Document style guide for docs

[davidmirror-ops]

What happened?

While it seems that documentation site and Github repos use a style guide, this is not documented explicitly neither at the site or Github

• What did you expect to happen?
  Documenting the style guide could be helpful for individuals willing to contribute to documentation/site content

• Previous art
  Velero style guide adapted from the Kubernetes one: https://github.com/vmware-tanzu/velero/blob/main/site/content/docs/v1.5/style-guide.md

[fg-j]

I would love love LOVE to see this for our project!