[docs] We should document the possible values for the icon attribute of Elements, like <rh-tag>, etc.

[markcaron]

Our docs aren't very clear on what string goes into the icon attribute on our Elements.

For example, on Tag:

The presumption is the names of the icons from <pf-icon> or <rh-icon>, but what are they?

[bennypowers]

For reference, https://patternflyelements.org/components/icon/demo/sets/ lists all the available icons

[hellogreg]

Tackle after rh-icon

[bennypowers]

there's some preliminary work in #1859 and #1844, just adding the correct types to the element source files

we can go back later and transform those types in 11ty into links to the icon page (#1844)

[bennypowers]

pushing the second phase of this (transforming the IconNameFor type in the 11ty attrs table to a link to iconography page) to cubone