Update turndown by implementing a custom rule

[MattiSG]

The dependency used to convert HTML into Markdown, turndown, is loaded from a fork to circumvent #903.

However:

According to mixmark-io/turndown#419 (comment) 1361030545, it is very unlikely OpenTermsArchive/turndown#2 will ever be merged into upstream.

The alternative suggested by the library author is to use a custom rule. This would indeed be a more perennial approach than maintaining our own fork.

(source: #903 (comment))

This means that until a custom rule is added and the original dependency is brought back, we are stuck with maintaining our own fork.