Document the paths that are or are not shared with the container
Before rethinking which paths should and should not be shared under which circumstances as requested on https://github.com/libretro/RetroArch/issues/14266, it's useful to document what currently happens, so that we have a framework in which we can make these decisions.
Helps: steamrt/tasks#149
HTML preview: https://gitlab.steamos.cloud/steamrt/steam-runtime-tools/-/blob/wip/smcv/task149/docs/shared-paths.md
Edited by Simon McVittie