Add NEP1 to documentation.

Changes:
  - Added NEP 1 to the documentation set.
  - Modified nimdoc's css to add additional spacing between list entries, as well as between code blocks.
  - Added a warning to nimdoc.css for future developers.

web/documentation.txt

@@ -14,6 +14,9 @@ Nim's Documentation
     - | `Language Manual <docs/manual.html>`_
       | The Nim manual is a draft that will evolve into a proper specification.
 
+    - | `Nim Style Guide <nep1.html>`_
+      | The stylistic conventions that Nim's official projects adhere to.
+
     - | `Compiler User Guide <docs/nimc.html>`_
       | The user guide lists command line arguments, special features of the
         compiler, etc.