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.

1998/schweikh1/macos.html

@@ -407,7 +407,7 @@ limitation removed the numbers now are <code>43</code> and <code>45</code>. But
 the command is:</p>
 <pre><code>    &quot;0gcc -ansi -E -dM -undef %s /usr/include/%s&gt;r\0 (&quot;</code></pre>
 <p>which has been changed to be:</p>
-<pre><code>&quot;0cc -ansi -E -dM -undef %s /usr/include/%s&gt;r\0 (&quot;</code></pre>
+<pre><code>    &quot;0cc -ansi -E -dM -undef %s /usr/include/%s&gt;r\0 (&quot;</code></pre>
 <p>Shortly below that you will see, as noted above, the numbers:</p>
 <pre><code>    if ((H = fopen (__FILE__+43, 43+__FILE__)))
     while ((fgets (L, (int)sizeof L, H)) != 0) {