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.
Triggered by the deployment failure that was caused by an invisible Unicode character in a rule's metadata.json.
This PR implements three conceptual changes:
- make the deployment parse error more informative
- prohibit the use of non-ASCII characters in the metadata.json files
- remove the existing non-ASCII characters from the existing rules
