Document how to use steam-runtime-launch-client in practice

Should not be merged until !465 (merged) and preferably steamlinuxruntime!76 (merged) are in releases (they should be in this week's beta container runtime updates, see steamrt/tasks#137). Ideally this wants https://github.com/ValveSoftware/Proton/pull/5891 too.

HTML preview: https://gitlab.steamos.cloud/steamrt/steam-runtime-tools/-/blob/wip/smcv/doc-launcher-service/docs/slr-for-game-developers.md