Expand CSS style guide to cover more conventions and recommendations (mdn#40228)

* Expand CSS style guide to cover more conventions and recommendations

* Add guide for casing

* Update index.md

* Suggestions

* Format

* Edits

* Apply suggestions from code review

Co-authored-by: Onkar Khadangale <87750369+OnkarRuikar@users.noreply.github.com>

* Update index.md

* Update index.md

* Update files/en-us/mdn/writing_guidelines/code_style_guide/css/index.md

* Update index.md

* Apply suggestions from code review

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Co-authored-by: Onkar Khadangale <87750369+OnkarRuikar@users.noreply.github.com>

Author: Josh-Cena