AsciiDoctor generates _ids_with_underscores

[ghost]

To allow for better SEO, we have gotten rid of all IDs with _ and . characters in them. However, when AsciiDoctor automatically generates IDs, they look like _chapter_title (i.e. an underscore at the beginning and underscores in between words).

We should (probably) convert all IDs that start with underscores automatically via the ADOC_POST stylesheet.