docs: Document some of the assumptions made by the Steam Runtime

In typical nearly-FHS OS distributions, most or all of this is trivially true, but in distributions with a more unusual layout such as NixOS, that isn't necessarily the case.

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Prompted by https://github.com/ValveSoftware/Dota-2/issues/2018.

This could be argued to be appropriate for either this repository, or https://github.com/ValveSoftware/steam-runtime, both of which are public and open-source.

I think steam-runtime-tools is a better home for this stuff, for several reasons:

• it's the same source tree as the pressure-vessel container launcher, so when we change container assumptions, we can change the documentation of those assumptions in the same commit
• it's the same source tree as the steam-runtime-system-info diagnostic tool, so when we change the documentation of assumptions, we can change the diagnostic tool in the same commit
• steam-runtime-tools has its own independent standalone releases, so changes here don't need to be kept in sync with the Steam internal-build/public-beta/release cycle, which is not under the control of the Steam Runtime developers
• steam-runtime has an inconveniently large history in .git for historical reasons (.deb packages were committed as binaries in the past), but this repository is purely source code and much smaller

Disadvantages of having it in this repository are:

• we can't currently accept pull requests from third-party contributors directly
  • this is not such a big problem, because people can open an issue and point to a clone on Github or elsewhere, and we can turn that into a local branch
• it isn't in the same repository as run.sh and setup.sh
  • this is less of a problem than not being in the same repo as pressure-vessel, because those scripts are slower-moving

After this has been merged, we should add a cross-reference in steam-runtime.

/cc @denittis @timo