Shorten CSS docs

Fold the two CSS chapters into one, make more compact
tables of selectors and properties, and stop explaining
general CSS syntax.

docs/reference/gtk/gtk4-docs.xml

@@ -371,7 +371,6 @@
   <part id="theming">
     <title>Theming in GTK</title>
     <xi:include href="css-overview.xml" />
-    <xi:include href="css-properties.xml" />
     <xi:include href="xml/gtkstylecontext.xml" />
     <xi:include href="xml/gtkcssprovider.xml" />
     <xi:include href="xml/gtkstyleprovider.xml" />

docs/reference/gtk/meson.build

@@ -343,7 +343,6 @@ content_files = [
   'building.xml',
   'compiling.xml',
   'css-overview.xml',
-  'css-properties.xml',
   'drawing-model.xml',
   'glossary.xml',
   'gtk4-broadwayd.xml',