Allow prefix `/` in autosummary's `toctree` to refer to base directory

Is your feature request related to a problem? Please describe.

At the moment, Autosummary's toctree directive is always relative to the current file. This means that the generated files always move with the file that contains an toctree-creating autosummary directive. In our case, we'd like to universally put the generated files into one separate folder at the documentation root, regardless of where they were generated from. This is particularly a problem when dealing with recursive autosummary generations, and at the moment is effectively limiting us to our manual documentation all having a flat folder structure.

Describe the solution you'd like

Perhaps a leading os.sep on the toctree directory could be interpreted as relative to the base documentation directory?

Describe alternatives you've considered

Nothing much short of leaving all our documentation in a flat structure / moving all the generated files to be next to the file that generated them.

Additional context

None.

(edit: I'd be happy to do this myself if it's an acceptable direction)

Start with the autosummary directive's toctree handling and how generated-file paths are resolved relative to the current file. Check how the base documentation directory is represented during recursive generation, then add coverage for a leading / while confirming ordinary relative paths remain unchanged.