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.
Inline adoc files when they are included exactly once.
Also fix language tags because this inlining gives us better information
on what language the code is written in.
