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/README.md

@@ -744,7 +744,7 @@ Keep in mind the way I display the generations and the fact that generation 0
 has no parents; it is generation 1 that starts working towards the goal. You'll
 see also **(mutation)** 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
-`` are separators: you won't see this between generation 0 and 1 but you will
+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 ``).