Add markdown files for documentation

projects.xml and declarative-projects.xml were merged with xmllint, and
then I ran that to convert files
for i in *.xml; do pandoc -s -f docbook -t markdown $i -o ${i/xml/md}; done

doc/manual/src/SUMMARY.md

@@ -0,0 +1,9 @@
+# Hydra User's Guide
+
+- [Introduction](introduction.md)
+- [Installation](installation.md)
+- [Creating and Managing Projects](projects.md)
+- [Using the external API](api.md)
+-----------
+[About](about.md)
+[Hacking](hacking.md)