Add section on icons to the DEV_GUIDELINES (#613)
Add section on icons to the DEV_GUIDELINES (#613)

# Description

- Felt like documenting this for future readers while the icon Design System updates are fresh in our mind.
chanzuckerberg/idseq-web,chanzuckerberg/idseq-web,chanzuckerberg/idseq-web,chanzuckerberg/idseq-web,chanzuckerberg/idseq-web