feat(Accordion): add icon shorthand for AccordionTitle

[sako9]

Added an example on how to add a custom icon via the standard Shorthand

[welcome]

💖 Thanks for opening this pull request! 💖

Here is a list of things that will help get it across the finish line:

• Run yarn lint locally to catch formatting errors. This will fix some errors automatically, commit and push any changes.
• Run yarn test locally to catch errors. This ensures all components still behave as they should.
• Run yarn start to run the doc site locally and try a few pages, ensuring everything is in good working order.
• Include tests when adding/changing behavior.

We get a lot of pull requests on this repo, so please be patient and we will get back to you as soon as we can.

[codecov-io]

Codecov Report

Merging #3417 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3417      +/-   ##
==========================================
- Coverage   99.92%   99.89%   -0.04%     
==========================================
  Files         172      172              
  Lines        2837     2835       -2     
==========================================
- Hits         2835     2832       -3     
- Misses          2        3       +1

Impacted Files	Coverage Δ
src/modules/Dropdown/Dropdown.js	99.76% <0%> (-0.24%)	⬇️
src/modules/Checkbox/Checkbox.js	100% <0%> (ø)	⬆️

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[sako9]

Looks good!

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉

[levithomason]

Released in [email protected].