docs: add events API to README

[ferreiro]

Improving docs with the new Events API + reorder some sections

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Note

Adds Events API docs and multi-select contact property guidance, updates examples, and bumps version to 1.19.0.

• Documentation (README)
  • Events API: Add section with single and batch send examples, error handling, and event schema (event, identify, properties, timestamp, messageId).
  • Contact Properties: Document multi-select behavior (arrays replace all values; add/remove for granular updates; empty array clears).
  • Minor content tweaks: improve batch contacts example formatting/perf note; simplify reply-to example; small copy edits in feature list.
• Examples
  • Update examples/contact-properties.ts to show multi-select replace and granular control functions, plus notes on new behavior.
• Release
  • Bump version to 1.19.0 in package.json and lockfile.

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