docs/user: Use Doxygen's internal search system.

Fixes #17209.

docs/user/Doxyfile

@@ -1850,7 +1850,7 @@ MATHJAX_CODEFILE       =
 # The default value is: YES.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-SEARCHENGINE           = NO
+SEARCHENGINE           = YES
 
 # When the SERVER_BASED_SEARCH tag is enabled the search engine will be
 # implemented using a web server instead of a web client using JavaScript. There

docs/user/book.css

@@ -231,12 +231,6 @@ h4 a:hover, a:active, h5 a:hover, a:active, h6 a:hover, a:active {
 	font-weight: normal;
 }
 
-#banner span.search {
-	position: relative;
-	top: 48px;
-	left: 600px;
-}
-
 input[type=text], input[type=password] {
 	font-size: inherit;
 	border-top: 1px solid #818181;

docs/user/header.html

@@ -22,9 +22,6 @@
 			<span class="subtitle">
 				API Documentation
 			</span>
-			<span class="search">
-				<input type="text" placeholder="Search" onkeydown="search(this,event);"/>
-			</span>
 		</div>
 	</div>
 	<div id="top"><!-- do not remove this div, it is closed by doxygen! -->