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.

2018/ferguson/index.html

@@ -895,7 +895,9 @@ bash shell built-in utility: that isn’t output from my entry.</p>
 has no parents; it is generation 1 that starts working towards the goal. You’ll
 see also <strong>(mutation)</strong> at the end of the line at this point (if you enable
 monkey mode you’ll find that it shows something else). The lines that only have
-<code>are separators: you won't see this between generation 0 and 1 but you will between future generations - as well as the final answer (if the answer is in generation 0 you won't see the</code>).</p>
+back-quote (“`”) are separators: you won’t see this between generation 0 and 1 but you will
+between future generations - as well as the final answer (if the answer is in
+generation 0 you won’t see the ``).</p>
 <div id="test">
 <p>One final thing to consider is that <strong>the program doesn’t directly read from a
 file</strong>. This combined with the way the shell works could possibly confuse some