They sure are. In hindsight the title is a little ambiguous but OSS is the standard for this type of developer tooling.

Core redocly is free. To have any more serious customization, you need to pay significantly.

Also, I highly appreciate redoclys starter template which tackles more serious topic about developing API first with reusable components instead of writing everything in one big file is is nightmare.

There's Widdershins plus Slate too. Widdershins turns the OpenAPI specs into markdown, then Slate into HTML. The results look really nice, and the use of templates means it's easy to customise. I'm just finishing my first project with them now, and I'd happily use them again.