Various small corrections to server documentation (#2050)

* Link to super from Handler and OperationService

* Note that structure documentation is from model

* Don't refer to ZSTs in operation_shape.rs opener

* Re-export everything from service to root

* Add reference to root documentation on service

* Fix spelling of MissingOperationsError

* #[doc(hidden)] for opaque_future

* Rename from-parts.md to from_parts.md

* Fix Connected link

* Use S type parameter and service as variable name

* Remove MissingOperation dead code

* Remove Operation header from operation.rs

* Remove reference to closures

* Document ConnectInfo<T> .0

* Add BoxBody documentation

* Rephrase operation.rs documentation

* Reference PluginPipeline from PluginStack

* Move the BoxBody documentation

* Document Plugin associated types

* Add example implementation for Plugin

* Link to plugin module from Plugin

* Improve FromParts/FromRequest documentation

* Remove links to doc(hidden) RuntimeError

* Add link from Upgradable to operation module