update: CONTRIBUTING guide to document process for adding new icons (#1939)

* update: CONTRIBUTING guide to document process for adding new icons

Added instructions for adding new icons to the existing font family, including steps for using Icomoon and updating the style.css file.

* Update CONTRIBUTING.md

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* Update CONTRIBUTING.md

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* Update CONTRIBUTING.md

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* Fix typos and improve clarity in CONTRIBUTING.md

Corrected capitalization of 'Unicode' and improved clarity in instructions for adding icons.

* Update CONTRIBUTING.md

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* Update CONTRIBUTING.md

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* Update CONTRIBUTING.md

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* update: renumber steps for clarity in CONTRIBUTING.md

* Update CONTRIBUTING.md

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* Update CONTRIBUTING.md

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