Generated documentation for the SDK

[eli-darkly]

Although the SDK contains documentation comments (in the TypeScript declarations), they haven't been viewable anywhere except in IDEs. We are planning to put TypeDoc-generated docs online. If you'd like to watch this issue, we will update it when the docs go online.

We've also done some editing to improve the completeness and correctness of these comments.

[eli-darkly]

TypeDoc documentation is now published at https://launchdarkly.github.io/node-server-sdk/ for the most recent release (5.8.0), and will be updated with each release. We don't yet have versioned documentation, but that's on the list.