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/burton2/README.md

@@ -343,7 +343,7 @@ NB: The keyword file used in this code is derived from the list in [iocccsize.c]
 which is neither complete (`#define`, `#ifndef`, `#undef` are missing
 
 ```
-yes, Virginia knows about `#define` omitted on purpose
+    yes, Virginia knows about `#define` omitted on purpose
 ```
 
 nor correct (many more are added: `I`, `true`, `bool`, `compl`, ...):