[css-gaps-1] Fixes for gap intersection point definition and pairing algorithm.

[kbabbitt]

• Revert commit f8b28b2.
• Update illustration for gap intersection points with spanning items in grid.
• Fix "determine pairs of gap intersection points" algorithm to apply gap-rule-break as intended.
• Minor markup enhancements.

[kbabbitt]

Formatted preview here: https://kbabbitt.github.io/css-gap-decorations/pr-11720/Overview.html

@oSamDavis I can't add you as a "Reviewer" for some reason but could you ptal?

[oSamDavis]

I wonder if it'll be helpful to add which intersection points are considered for -rule-break: intersection. So maybe along the lines of "Gap decorations start and end at every valid gap intersection point. A valid gap intersection point intersection is defined as one that forms a visible cross or T intersection." then maybe a "refer to the steps below."

[kbabbitt]

Agree that the value definitions need to be updated, but I'd prefer not to introduce another new term ("valid") if we can avoid it. I changed the value definitions to be based around visible intersections and added a link to the algorithm steps for precise details. I also reordered the value definitions here to match the order they appear in the algorithm. Let me know what you think.

[oSamDavis]

Just took a look and it looks great!

[oSamDavis]

New edits look good to me. Thanks!