[css-grid-3] Add more subheadings and links for alignment

Author: fantasai

css-grid-3/Overview.bs

@@ -1078,14 +1078,27 @@ Sizing Grid Containers</h2>
 <h2 id="alignment">
 Alignment and Spacing</h2>
 
-	[[css-grid-2#gutters|Gutters]] are supported in both axes.
+<h3 id="gutters">
+Gutters: the 'row-gap', 'column-gap', and 'gap' properties</h3>
+
+	[[css-grid-2#gutters|Gutters]] are supported in both axes
+	using the 'row-gap' and 'column-gap' properties (and their 'gap' [=shorthand=]).
+	In the [=grid axis=] they work the same as in a regular [=grid container=];
+	see [[css-grid-2#alignment]].
 	In the [=stacking axis=],
 	the gap is applied between the margin boxes of each pair of adjacent items.
 	Margins do not collapse in either axis.
 
+<h3 id="grid-axis-alignment">
+Grid-axis Alignment: the 'align-content'/'justify-content', 'align-self'/'justify-self', and 'align-items'/'justify-items' properties</h3>
+
 	In the [=grid axis=],
-	[[css-grid-2#alignment|alignment]]
-	works the same as in a regular [=grid container=].
+	the [=box alignment properties=]
+	work the same as in a regular [=grid container=].
+	See [[css-grid-2#alignment]] and [[css-align-3|CSS Box Alignment Level 3]].
+
+<h3 id="stacking-content-alignment">
+Stacking-axis Content Distribution: the 'align-content'/'justify-content' properties</h3>
 
 	In the [=stacking axis=],
 	[[css-align-3#content-distribution|content-distribution]] is applied
@@ -1119,6 +1132,9 @@ Alignment and Spacing</h2>
 	the [=grid container=]'s [=content box=] in the [=stacking axis=],
 	then the [=stacking range=] will be larger than the [=grid container=]'s [=content box=].
 
+<h3 id="stacking-self-alignment">
+Stacking-axis Self Alignment: the 'align-self'/'justify-self' and 'align-items'/'justify-items' properties</h3>
+
 	In the [=stacking axis=],
 	the [=self-alignment properties=] only apply to items that are adjacent to a gap in the layout,
 	i.e. placed in their [=grid track=](s) either immediately before a spanning item
@@ -1137,7 +1153,6 @@ Alignment and Spacing</h2>
 
 	ISSUE(10275): Is this a reasonable definition for how the [=self-alignment properties=] should work in the [=stacking axis=]?
 
-
 <h3 id="grid-lanes-baseline-alignment" oldids="masonry-baseline-alignment">
 Baseline Alignment</h3>