Run Pandoc manually until Hugo has better native support:

See relevant Hugo issues/PRs:

• better pandoc support gohugoio/hugo#8152
• Allow configuring Pandoc gohugoio/hugo#7529
• Add basic toc generation for pandoc gohugoio/hugo#8911
• Passing argument to external package from user input or hugo config file instead of hardcode in hugo gohugoio/hugo#9351