build: move the syntax files out of the doc/ directory

They are not documentation, they are functional elements of nano.

syntax/java.nanorc

@@ -0,0 +1,16 @@
+## Here is an example for Java.
+
+syntax "java" "\.java$"
+magic "Java "
+comment "//"
+
+color green "\<(boolean|byte|char|double|float|int|long|new|short|this|transient|void)\>"
+color red "\<(break|case|catch|continue|default|do|else|finally|for|if|return|switch|throw|try|while)\>"
+color cyan "\<(abstract|class|extends|final|implements|import|instanceof|interface|native)\>"
+color cyan "\<(package|private|protected|public|static|strictfp|super|synchronized|throws|volatile)\>"
+color red ""[^"]*""
+color yellow "\<(true|false|null)\>"
+color blue "//.*"
+color blue start="/\*" end="\*/"
+color brightblue start="/\*\*" end="\*/"
+color ,green "[[:space:]]+$"