[docs] Icon page update

[marionnegp]

Description

This is an update to the formatting and copy for the Icon page made in #1844.

What do you think about using this layout? It keeps the icons pretty dense, but using link button without the icon inside the button helps with scanning imo. I also bumped up the size of the icons to make them easier to see.

Other changes:

• I think this should be added to the "Foundations" section. I'm not sure this warrants being a top-level page, and PatternFly also has an "Icons" page under their "Design foundations" section.
• Added an "Overview" section with info about how to use the buttons on the page and links to <rh-icon> docs.
• Removed copy under each section heading. I'd like to avoid having to update usage info on two different pages.
• Added cross links that most of our pages have at the bottom.

Links

No response

Screenshots

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Additional context

@bennypowers, feel free to unassign yourself if you have too much on your plate! I only added you because you made the original version.

@markcaron @hellogreg, I added this to the Clefairy milestone to update what's already been merged to staging/clefairy.