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.

2020/ferguson1/gameplay.html

@@ -615,8 +615,8 @@ might look like (I set the starting size to 150 and had only just started):</p>
     #                                                #
     ##################################################</code></pre>
 <p>If I continue going up I will see:</p>
-<pre><code>OUCH!
-X:20/49 Y:1/16 S:150/658 B:0</code></pre>
+<pre><code>    OUCH!
+    X:20/49 Y:1/16 S:150/658 B:0</code></pre>
 <div id="cannibalism">
 <h1 id="gameplay-cannibalism"><a href="#toc">Gameplay: Cannibalism</a></h1>
 </div>

2020/ferguson1/gameplay.md

@@ -294,8 +294,8 @@ might look like (I set the starting size to 150 and had only just started):
 If I continue going up I will see:
 
 ```
-OUCH!
-X:20/49 Y:1/16 S:150/658 B:0
+    OUCH!
+    X:20/49 Y:1/16 S:150/658 B:0
 ```