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

[jakelishman]

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)