Improve package metadata and ecosystem discoverability

[woksin]

Improves how Arc and its packages are discovered on NuGet, npm, GitHub, and search engines, and cross-links the wider Cratis ecosystem. Documentation and metadata only — no behavior changes.

Added

• NuGet <Description> for Cratis.Arc, Cratis.Arc.Core, Cratis.Arc.Chronicle, Cratis.Arc.MongoDB, Cratis.Arc.EntityFrameworkCore, Cratis.Arc.OpenApi, Cratis.Arc.Swagger, both analyzer packages, and both proxy generator packages, plus a default overridable <PackageTags> set and per-package tags
• npm description, keywords, and homepage for @cratis/arc, @cratis/arc.react, @cratis/arc.react.mvvm, and @cratis/arc.vite
• README links to the "Why Arc" and "Coming from MediatR and MVC" documentation pages and to the runnable samples
• README section describing the wider Cratis ecosystem with links to Chronicle, its language clients, Components, the CLI and Workbench, and supporting projects
• Documentation landing cards for Why Arc, Coming from MediatR and MVC, and Samples

Changed

• README tagline now states that Arc is open source (MIT), works without event sourcing, and integrates optionally with Chronicle