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.

next/guidelines.html

@@ -25,7 +25,7 @@
 <!-- END: this line ends content from: inc/head.default.html -->
 
 <!-- -->
-<!-- This web page was formed via the tool: bin/gen-status.sh -->
+<!-- This web page was formed via the tool: bin/gen-top-html.sh -->
 <!-- The content of main section of this web page came from: next/guidelines.md -->
 <!-- -->
 

next/rules.html

@@ -25,7 +25,7 @@
 <!-- END: this line ends content from: inc/head.default.html -->
 
 <!-- -->
-<!-- This web page was formed via the tool: bin/gen-status.sh -->
+<!-- This web page was formed via the tool: bin/gen-top-html.sh -->
 <!-- The content of main section of this web page came from: next/rules.md -->
 <!-- -->