Documentation fixes

+ Rename some txt2tags file from .txt to .t2t and remove abandoned .txt files.
+ Add program update_html that finds all .t2t documents and updates the
  corresponding .html file. It can be invoked with 'make html'.
+ Add style to some .html documents

css/style.css

@@ -1,7 +1,9 @@
 
 body { background-color: #f2f2f2; }
 
+
 h1,h2,h3 { font-family: sans-serif; }
+h1 { text-align: center; }
 h2 { border-bottom: 2px solid black; clear: right; }
 
 div.center { text-align: center; }