Improve discoverability: event-sourcing positioning, badges, and ecosystem cross-links

[woksin]

What changed

• README: added an "Event sourcing for Elixir" tagline, Hex.pm / HexDocs / MIT badges, and an Install section with the current ~> 2.2 dependency snippet; fixed the broken Documentation/getting-started.md link (the file is get-started.md); appended a "The Cratis ecosystem" section cross-linking Chronicle, the other Chronicle clients (.NET, TypeScript, Kotlin/Java; Python coming soon, pre-alpha), the Chronicle MCP server, CLI + Workbench, Arc/Components, and Samples.
• Documentation/get-started.md: reconciled the install constraint ~> 0.1 with the versions actually published on Hex (latest 2.2.0) — the old constraint would not resolve to any current release.
• Source/chronicle/mix.exs: extended the Hex package links map with Chronicle, the Chronicle docs on cratis.io, and cratis.io, so the Hex.pm page points people at the wider ecosystem.

Why

Better discoverability: people searching for event sourcing / CQRS / event store for Elixir should find this client, understand at a glance what it is and that it is MIT licensed and free, and be able to navigate to the rest of the Cratis stack. No code changes.