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.

bugs.html

@@ -1370,25 +1370,25 @@ original code has had a buffer size increase.</p>
 comment as well as the <code>int r=0,x,y=0</code> at the top of the file. If you look at
 each column go down that column you can see how it spells out the code! For
 instance the first column looks like:</p>
-<pre><code>i
-n
-c
-l
-u
-d
-e
+<pre><code>    i
+    n
+    c
+    l
+    u
+    d
+    e
 
-&lt;
-s
-t
-d
-i
-o
-.
-h
-&gt;</code></pre>
+    &lt;
+    s
+    t
+    d
+    i
+    o
+    .
+    h
+    &gt;</code></pre>
 <p>If you join the lines you end up with:</p>
-<pre><code>#include &lt;stdio.h&gt;</code></pre>
+<pre><code>    #include &lt;stdio.h&gt;</code></pre>
 <p>If you look at column 25 which is the end of the word ‘mh111’ and you go down to
 the next row you’ll see a 0 and if you go one row down another 0. This is the
 buffer size, 100, for <code>u</code>! The column to the left is the same for the <code>t</code>