fix non-indented markdown code blocks

We have fixed cases where markdown code blocks were not properly intended.

Updated `markdown.md` to discuss how markdown code blocks should be indented
and how blank lines should follow a markdown code block.

We used this shell script to find problematic code blocks:

```sh
find . -name '*.md' ! -name markdown.md -print | xargs pcregrep -M '^``.*\n[^|`\n \t]'
```

Ran `make www` to perform the above.

2013/hou/README.md

@@ -161,7 +161,7 @@ Despite the messy look, `a.c` and [hou.c](%%REPO_URL%%/2013/hou/hou.c) compile w
 `a.c` compiles mostly clean in the C99/ANSI modes of clang and gcc (with:
 
 ```
--Wall --pedantic
+    -Wall --pedantic
 ```
 
 The only warning generated is a pedantic one: `"string constant