EDU-17673 - Add guide about the Events API

[mariananantua]

Summary

Add guide about the Weni Events API. Refers to EDU-17673.

This PR is linked to two others:

• Add Weni Events API schema to openapi-schemas repo.
• Update navigation.json in the devportal repo.

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Type of change

• New content — Adds new documentation.
• Content update — Improves existing documentation (clarity, structure, examples, accuracy).
• Bug fix — Fixes markdown issues.
• Editorial fix — Spelling, grammar, or minor copy edits that don’t change meaning.
• Content removal — Deletes deprecated or obsolete content.

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Checklist

• The content follows the VTEX Style Guide.
• Markdown renders correctly (headings, lists, tables, callouts).
• Links, images, code blocks, and examples are valid.
• Terminology, product names, and API references are consistent.
• Frontmatter (title, description, tags, slug) is correct
• Files follow the expected folder structure and naming conventions.