docs: describe workshop audience and link the wider Cratis docs and samples

[woksin]

Summary

Part of the org-wide discoverability pass. Small, proportionate README update for this supporting repo:

• Adds a Who these workshops are for section (audience: developers learning event sourcing and CQRS with the Cratis stack; no prior Cratis experience required).
• Notes in the workshop table that the EventModelingConference2026 workshop runs against MongoDB (default), PostgreSQL, SQL Server, or SQLite through Chronicle's pluggable storage providers — verified against Core/run.sh and the workshop README.
• Adds a Learn more section linking the Cratis docs, the Chronicle and Arc repositories and docs, and the Samples repository.
• States the MIT/free licensing fact.

No claims beyond what the repository source verifies.