[css-grid] Combine implicit + explicit grid secitons

Author: fantasai

css-grid/Overview.bs

@@ -1078,6 +1078,9 @@ Implied Minimum Size of Grid Items</h3>
 		this traversal is trivial and therefore not a performance concern.)
 	</div>
 
+<h2 id='grid-definition'>
+Defining the Grid</h2>
+
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@@ -1088,8 +1091,8 @@ Implied Minimum Size of Grid Items</h3>
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-<h2 id='grid-definition'>
-The Explicit Grid</h2>
+<h3 id='explicit-grids'>
+The Explicit Grid</h3>
 
 	The three properties 'grid-template-rows', 'grid-template-columns', and 'grid-template-areas'
 	together define the <dfn export local-lt="explicit">explicit grid</dfn> of a <a>grid container</a>.
@@ -1113,7 +1116,7 @@ The Explicit Grid</h2>
 	(starting from -1 for the <a>end</a>-most <a>explicit</a> line).
 
 <h3 id='track-sizing'>
-Track Sizing: the 'grid-template-rows' and 'grid-template-columns' properties</h3>
+Explicit Track Sizing: the 'grid-template-rows' and 'grid-template-columns' properties</h3>
 
 	<pre class='propdef'>
 	Name: grid-template-columns, grid-template-rows
@@ -1983,8 +1986,8 @@ Explicit Grid Shorthand: the 'grid-template' property</h3>
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 -->
 
-<h2 id='implicit-grids'>
-The Implicit Grid</h2>
+<h3 id='implicit-grids'>
+The Implicit Grid</h3>
 
 	The 'grid-template-rows', 'grid-template-columns', and 'grid-template-areas' properties define a fixed number
 	of tracks that form the <a>explicit grid</a>.
@@ -2062,7 +2065,7 @@ The Implicit Grid</h2>
 -->
 
 <h3 id='auto-tracks'>
-Sizing Auto-generated Rows and Columns: the 'grid-auto-rows' and 'grid-auto-columns' properties</h3>
+Implicit Track Sizing: the 'grid-auto-rows' and 'grid-auto-columns' properties</h3>
 
 	<pre class='propdef'>
 	Name: grid-auto-columns, grid-auto-rows
@@ -2281,8 +2284,8 @@ Sizing Auto-generated Rows and Columns: the 'grid-auto-rows' and 'grid-auto-colu
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 -->
 
-<h2 id='grid-shorthand'>
-Grid Definition Shorthand: the 'grid' property</h2>
+<h3 id='grid-shorthand'>
+Grid Definition Shorthand: the 'grid' property</h3>
 
 	<pre class='propdef'>
 	Name: grid