Split server and archive writer

For @quietmisdreavus and rustdoc integration.

- [ ] Add two cargo features: `build`, and `serve`, both enabled by default
- [ ] Further split up `lib.rs` and conditionally enable stuff based on the set features
- [ ] Remove quicli dependency and instead add only the stuff we use

Start by reading lib.rs and the existing Cargo feature and dependency declarations to understand how server and archive-writing paths are currently combined. Separate the build and serve feature paths, keep both enabled by default, and verify that the quicli dependency is removed while the needed functionality remains available.

Written by the indexing model from the issue text.