Standardize citation style in function documentation

[gemini-wenb]

Some functions include references to journal publications or CIHI documents. This is currently inconsistently done across functions.

Having a reference section with consistent style will facilitate researchers in easily identifying the correct citations when using our package functions.

This issue is to:

• add a @references section to functions needed
• use consistent reference style. A concise style will be used for keeping documentation simple: [Author, Year]. (doi (whenever possible)/url)
• add this as a review item to Code Review Checklist.