Rename user manual files

Use chapter ids instead of numbers, so that we can reorder them,
introduce new ones etc. without the numbers becoming out of date.

docs/Makefile.am

diff --git a/docs/Makefile.am b/docs/Makefile.am
index 64344b8..05a27db 100644
--- a/docs/Makefile.am
+++ b/docs/Makefile.am
@@ -71,12 +71,12 @@
 # Extra SGML files that are included by $(DOC_MAIN_SGML_FILE).
 # e.g. content_files=running.sgml building.sgml changes-2.0.sgml
 content_files=	\
-	usermanual-ch01.xml \
-	usermanual-ch02.xml \
-	usermanual-ch03.xml \
-	usermanual-ch04.xml \
-	usermanual-ch05.xml \
-	usermanual-ch06.xml \
+	usermanual-buffers-language-script-and-direction.xml \
+	usermanual-fonts-and-faces.xml \
+	usermanual-glyph-information.xml \
+	usermanual-hello-harfbuzz.xml \
+	usermanual-opentype-features.xml \
+	usermanual-what-is-harfbuzz.xml \
 	version.xml
 
 # SGML files where gtk-doc abbrevations (#GtkWidget) are expanded