Add microdata markup to generated html output

[berezinant]

Provided with microdata markup dokka's html pages could gain several improvements:

• better indexing by search engines and possibly for AI — seems rather useful and could help users find better answers about Kotlin libraries and their API.
• previews for documentation pages in social media — this use case seems to be rare but definitely there is no harm in it

The implementation could be done with OG (open graph), schema.org tags and/or other similar tools