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

2013/hou/index.html

@@ -507,7 +507,7 @@ Despite the messy look, <code>a.c</code> and <a href="https://github.com/ioccc-s
 (<a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2013/hou/hou.c">hou.c</a> even wastes 18 bytes on <code>#include&lt;stdio.h&gt;</code> just for <code>putchar(3)</code>).
 <code>a.c</code> compiles mostly clean in the C99/ANSI modes of clang and gcc (with:</li>
 </ul>
-<pre><code>-Wall --pedantic</code></pre>
+<pre><code>    -Wall --pedantic</code></pre>
 <p>The only warning generated is a pedantic one: <code>"string constant too long"</code>.</p>
 <h3 id="comments-and-why-obfuscated">Comments and why obfuscated</h3>
 <ul>