In some cases, the `rule.adoc` at root of a rule is never included
anywhere and thus is dead code.
It's a maintenance cost by itself, but also it misses opportunities to
inline code that seems used by two documents when in fact only one
document is actually rendered. And this missed opportunity, in turn,
stops us from applying the correct language tag on the code samples.
When an include is not surrounded by empty lines, its content is inlined
on the same line as the adjacent content. That can lead to broken tags
and other display issues.
This PR fixes all such includes and introduces a validation step that
forbids introducing the same problem again.
