Document engine runtime dependencies and options

Currently there's no unified resource on what the engine expects to be present to minimally function, and what those requirements/dependencies are, examples of things we need to have available:

- Engine binary `spring` usage, options and input arguments (e.g. startscript)
- What kind of external dependencies to the engine binary are required or at least required to be known, e.g. basecontent, binary only works inside engine distribution folder or not, path to resource assets dir (writedir, datadir, etc)
- Anything else that is relevant for games using the engine at a lower level (e.g. developing lobby clients or hosting mechanisms) or engine developers to understand what to expect and how to configure their build/debug/release/run targets/workflows.

Start by examining the engine binary `spring`, including its usage, options, and startscript input arguments. Identify the runtime requirements around basecontent, the engine distribution folder, and resource paths such as writedir and datadir, then document the expectations and configuration relevant to game, lobby, hosting, and engine workflows.

Written by the indexing model from the issue text.