Remove unused SearchState and SidebarCollapseState #46

Summary

Search and sidebar collapse moved to shelldocs.js (they work without a Blazor runtime), so SearchState and SidebarCollapseState are no longer used, but AddShellDocs still registers them.

Task

• Delete src/ShellDocs.Components/Chrome/SearchState.cs and SidebarCollapseState.cs
• Remove their registrations from ServiceCollectionExtensions.cs
• Check nothing else references them, build with 0 warnings and run the tests
• Note the removal in CHANGELOG.md (public types, so it's a breaking change for anyone injecting them)