Use more common web-style paragraphs for the docs

See <http://www.naturaldocs.org/styles.html#WebStyleParagraphs>.

docs/naturaldocs/project/libgd.css

@@ -0,0 +1,9 @@
+/*
+Custom styles for the generated HTML, which are supposed to override the default
+styles
+*/
+
+p {
+    text-indent: 0;
+    margin-bottom: 1em;
+}

docs/naturaldocs/run_docs.sh

@@ -68,7 +68,8 @@ sed -e "s/@VERSION@/$VERSION/g" preamble.txt > tmp/preamble.txt
 $(nd) --rebuild --rebuild-output --documented-only \
     -i tmp/ \
     -o html html  \
-    --project project/
+    --project project/ \
+    -s Default libgd
 
 # And cleanup the temp files.
 rm -rf Data tmp