Added an importing icons section in CONTRIBUTING.md with the process of adding/using new icons in packages/iTwinUI-react.
Removed importing icons styling recommendations in STYLEGUIDE.md since it is partially included in CONTRIBUTING.md (#991 (comment))
Old PR description before https://github.com/iTwin/iTwinUI-react/pull/991#discussion_r1047682881
* Updated `STYLEGUIDE.md` to recommend importing icons from `src/core/utils` in `packages/itwinui-react` (new recommendation) and import from cjs in other modules (existing recommendation).
* Added styling recommendations for imports for illustrations to import illustrations similar to icons.
* Corrected incorrectly imported icons and illustrations, according to the style guide.
Checklist
~[ ]~ Add meaningful unit tests for your component (verify that all lines are covered)
[x] Verify that all existing tests pass
~[ ]~ Add component features demo in Storybook (different stories)
~[ ]~ Approve test images for new stories
~[ ]~ Add screenshots of the key elements of the component
Closes #925
CONTRIBUTING.mdwith the process of adding/using new icons inpackages/iTwinUI-react.STYLEGUIDE.mdsince it is partially included inCONTRIBUTING.md(#991 (comment))Old PR description before https://github.com/iTwin/iTwinUI-react/pull/991#discussion_r1047682881
* Updated `STYLEGUIDE.md` to recommend importing icons from `src/core/utils` in `packages/itwinui-react` (new recommendation) and import from cjs in other modules (existing recommendation). * Added styling recommendations for imports for illustrations to import illustrations similar to icons. * Corrected incorrectly imported icons and illustrations, according to the style guide.Checklist