add abilty to control icon button sizes independent of button size

[steppy452]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Currently, if we set buttons to have unequal x and y padding, you would have to always add styles (or a custom size) to icon buttons if you wanted to make to square.

Issue Number: N/A

What is the new behavior?

Since we have an independent <IconButton /> component, we should allow for the icon button size to be controlled independently. This PR adds theme to allow control of icon button sizes independently of regular button sizes.

Also added a new story to demonstrate:

Does this PR introduce a breaking change?

[ ] Yes
[x] No - setting default icon button size padding to be the same as default button padding

Other information

Docs update - reaviz/reablocks-website#35

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