shelldocs preview is listed but not implemented #43

Summary

shelldocs preview is a CLI command, but it only prints "not yet implemented".

Proposal

Either implement it, or remove it from the CLI until it exists. A useful version would serve the static output of shelldocs build (default publish/) on a local port, honouring --base-href, so a static export can be checked without Python or another server:

shelldocs build
shelldocs preview            # serves publish/ at http://localhost:5050
shelldocs preview --base-href /my-repo/

Where

src/ShellDocs.CLI/Program.cs (the preview command).