simplify experimental flags section

[sarah11918]

This makes the Experimental Flags section more concise for readability, plus adds:

• live collections
• a link for headingIdCompat

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[sarah11918]

(Requested @florian-lefebvre 's review since he's the one most affected here, and I added a link related to an earlier entry for him to verify! But, will gladly take other reviews!!)

[florian-lefebvre]

Love it!