Doc :read-write checkbox in "CSS features reference" (#3625)

* :read-write checkbox

* catch up outline

* update local toc

* fill PR's section

* inc rvw

* keep h2

* update local toc

* Update format-icon.png

* clean up todo cmts

* Update microsoft-edge/devtools/css/reference.md

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Co-authored-by: Patrick Brosset <patrick.brosset@microsoft.com>

Author: mikehoffms