Add guidance about creating and including snippets in docs for contributors.

## Context

To prevent duplication in docs we want to implement a snippets feature that meets our needs.

## Tasks

1. After #4556 is closed, document how to use the snippets plugin in the [docs contributor guide](https://github.com/knative/docs/tree/main/contribute-to-docs).

Start by checking issue #4556, then read the docs contributor guide at contribute-to-docs to identify where the snippets guidance belongs. Done means the guide explains how contributors create and include snippets using the completed snippets feature, without duplicating existing documentation.

Written by the indexing model from the issue text.