Fix docs build for website styles

Building the docs with STYLE=website referenced a stylesheet that long
longer exists on the website, since we changed it to use versioned
references.

To make it less likely for this to happen again, point to a single
stylesheet on the website which will in turn import the required one.
That puts the process entirely within the scope of the website
repository, so next time a version is switched that's the only place
changes have to be made, making them less likely to be missed.

Per (off-list) discussion with Peter Geoghegan and Jonathan Katz.

doc/src/sgml/stylesheet.xsl

@@ -24,10 +24,7 @@
   <xsl:choose>
     <xsl:when test="$website.stylesheet = 0">stylesheet.css</xsl:when>
     <xsl:otherwise>
-      https://www.postgresql.org/media/css/fontawesome.css
-      https://www.postgresql.org/media/css/bootstrap.min.css
-      https://www.postgresql.org/media/css/main.css
-      https://www.postgresql.org/media/css/normalize.css
+      https://www.postgresql.org/media/css/docs-complete.css
     </xsl:otherwise>
   </xsl:choose>
 </xsl:param>