Add 'hugo new content' instructions to contributing guidelines

[jputrino]

When creating a new document, contributors should run the hugo new content command instead of copying an existing doc. This ensures that the new document uses the correct frontmatter and template, and that metadata from other docs doesn't end up being duplicated (or triplicated!).

Definition of Done:

• Update the Add new documentation section of the CONTRIBUTING_DOCS guide to include instructions for running hugo new content.
• Must include examples that demonstrate how to format the desired file path and name for the new document; for example hugo new content <file path>/<file name>.md
• Must include examples for using the command to create different types of documents; for example, hugo new content <file path> -k concept