The API-reference generator needs a few updates to:

• Match the changes mentioned in API reference: typo in front matter and more website#330.
• Generate page front matter
• Reconsider where the output should go: currently, the script is targeting Documentation/dev-guide/api_concurrency_reference_v3.md. The website docs aren't under Documentation anymore. Either the output target should be changed, or instructions should be given to let the script user know where to copy/submit the files to.
• As a followup action, port to the release-3.5 branch

cc @nate-double-u @gyuho