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.

thanks-for-help.md

@@ -1506,6 +1506,7 @@ changed from `char*s` to `char s[]`):
 ```
     !.Xop.fssps!.Xop.sfuvso.uzqf!.Xop.jnqmjdju.gvodujpo.efdmbsbujpo
 ```
+
 and then the call to `system()` had to be changed to:
 
 ``` <!---c-->