Doc: Make \youtube macro work with litehtml help backend

Unlike the QTextBrowser backend, litehtml does not render elements
inside <iframe> correctly. This prevented external links to
YouTube from working in offline documentation.

Move the <iframe> to a macro override specific to online doc builds.

Fixes: QTBUG-88975
Change-Id: Iff7828ddeed353620eaa9ac669a3e0c03749daa2
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
(cherry picked from commit ca8e9ff2ef0a01248e3bdd718ef9fc06419214a4)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

doc/global/macros-online.qdocconf

@@ -0,0 +1,4 @@
+macro.youtube.HTML = "<div class=\"video\">\n<span class=\"vspan\"></span>\n" \
+                     "<iframe src=\"https://www.youtube.com/embed/\1\"" \
+                     "frameborder=\"0\" allowfullscreen>\n" \
+                     "</iframe></div>\n"

doc/global/macros.qdocconf

@@ -57,12 +57,10 @@ macro.QtMinorVersion.match  = "\\d+\\.(\\d+)"
 # qhp.ProjectName.extraFiles += images/dQw4w9WgXcQ.jpg
 #
 macro.youtube.HTML = "<div class=\"video\">\n<span class=\"vspan\"></span>\n" \
-                     "<iframe src=\"https://www.youtube.com/embed/\1\"" \
-                     "frameborder=\"0\" allowfullscreen>\n" \
                      "<a href=\"https://www.youtube.com/watch/?v=\1\">\n"\
                      "<img src=\"images/\1.jpg\"" \
                      "title=\"Click to play in a browser\" /></a>\n" \
-                     "</iframe></div>\n"
+                     "</div>\n"
 
 macro.excludefromcreator = "\\meta tag broken"
 

doc/global/qt-module-defaults-online.qdocconf

@@ -23,6 +23,9 @@ include(manifest-meta.qdocconf)
 include(fileextensions.qdocconf)
 include(qt-html-templates-online.qdocconf)
 
+#override macros for online use
+include(macros-online.qdocconf)
+
 #extra configuration data DITA information
 include(config.qdocconf)