Speed up site generation

Generating the documentation site is rather slow because oak-doc currently uses a parser that is not maintained anymore. See: https://github.com/sirthias/pegdown/

The deprecation note also mentions performance issues. My experience shows, the parser has difficulties with underscores in words. E.g. DOCUMENT_NS_BGW_NUM_WRITE_RATE is a name for a metric I wanted to document. Parsing time seems to increase exponentially the more such names appear in a markdown file.