Add/clarify documentation of enketo-express's public API

[eyelidlessness]

While several other Enketo projects are libraries, whose public interface is or may be inferred to be its publicly available exports, enketo-express is intended to be a set of backend endpoints and a web UI. We should clarify in documentation which enketo-express functionality is considered part of its public API:

• [x] Which backend APIs are public (versus those intended for use internally by the Enketo web UI). This appears to already exist, built from enketo/enketo-api-docs
• [ ] Which parts of the codebase, if any, is expected to be imported and consumed by other projects. While there are forks which consume various exports to provide the backend service and UI, enketo-express doesn't appear to be published to NPM, so it seems unlikely that there is any intent to provide it as a library.
• [ ] Which parts of the codebase can or should be expected to be stable for downstream use.
• [ ] Determine which parts of the codebase should be published to the project documentation, and filter out any which should not be