Add new "icons" column to terminology docs table.

[edonehoo]

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Follow-up to https://github.com/patternfly/patternfly-org/pull/4671

Add a new column to the UX writing terminology table for related icons. This will let us add icons by the recommended terms, so people don't have to flip back and forth between the icon guidelines. For example, "Add" might show the plus circle icon, "Cancel" the x icon, and so on.