Revamp Icon Design page

This is a big diff that implements the following changes:

• Move the individual icon types to sub-pages so this information isn't so overwhelming on a single page
• Create new margins graphics for each type of icon that are more clear and contextually-appropriate
• Be clearer about which sizes are used for each icon type and style
• Add real captions to images by using the .. figure:: tag
• Replace some images with better ones
• Improve wording in some places