Feature request: generate header comment on generated files

[kubukoz]

Sometimes it's not easy to notice at first sight that a file has been generated from another, especially if --check isn't in place.

It would be great to have some sort of signifier in the generated files, making it more obvious that they're not the source.

For example, something like:

<!-- ATTENTION: This is generated code. -->
<!-- This file has been generated by mdoc from the input file input/file.md. -->
<!-- Do NOT edit it manually, as it may get out of sync with the sources! Edit the input file instead. -->

at the beginning of each generated file, configurable via the usual config mechanism.