Clarify maintainers

There is a [Maintainers](https://www.hpc-carpentry.org/maintainers/) page, but nobody listed. The [current (commented out) mechanism](https://github.com/hpc-carpentry/hpc-carpentry.github.io/blob/61c79eb61505c326d08afaf0c2726efda28d1322/pages/maintainers.html#L21) for doing this does not easily translate to HPC Carpentry maintainers, so this might need to be done manually.