Naming conflict: `anvil/reference.md` and `anvil/reference/` folder

There is a naming conflict between `vocs/docs/pages/anvil/reference/` (folder) and `vocs/docs/pages/anvil/reference.md` (file), which makes the latest not visible in the navigation pane. I propose to move `reference.md` into the `reference/` folder (e.g., rename to `api.mdx`) and maybe refactor content in relation to `reference/anvil.mdx` to avoid duplication and improve structure. if you find this relevant, I'll make a PR 👍