docs: add Rekomi affiliate integration guide

[caldwellg]

📋 Description

Adds an integration guide for Rekomi (https://rekomi.com), an affiliate
tracking and management platform with a native Dodo Payments connection. I
emailed your support about a listing on July 14 and am following
CONTRIBUTING.md with a PR in the meantime.

What's included:

• integrations/rekomi.mdx, following the shape of the Dub guide: Steps
  setup, implementation examples for checkout sessions and static payment
  links, screenshot and logo committed in-repo.
• A new "Affiliates" group in the en navigation for it.
• A third Integration Option and a Rekomi section on features/affiliates.mdx,
  in the same summarize-and-link format as the Dub Partners section.

The connection itself is a single pasted full-access API key: Rekomi creates
the webhook endpoint via your API and fetches the signing secret itself, with
attribution carried as rekomi_ref checkout metadata (or the flat
metadata_rekomi_ref payment-link parameter).

🎯 Type of Change

• 📝 Documentation update (improving existing docs)
• ✨ New documentation (adding new guides, API docs, etc.)
• 🐛 Bug fix (fixing errors, broken links, incorrect information)
• 🎨 Style/formatting (improving readability, fixing formatting)
• 🏗️ Structural change (reorganizing content, navigation updates)
• 🌐 Translation (adding or improving translations)
• 🖼️ Assets (adding or updating images, diagrams)

📖 What documentation does this PR affect?

• Getting Started / Setup
• API Reference
• Developer Resources
• Integration Guides
• Features Documentation
• External Integrations
• Changelog
• Other:

✅ Checklist

• I have read the Contributing Guidelines
• I have tested my changes locally with mintlify dev
• All links are working correctly
• Images are properly displayed and optimized
• Code examples are tested and working
• I have updated the navigation in docs.json (if applicable)
• My changes follow the project's style guide
• I have performed a self-review of my changes
• I have added appropriate frontmatter to new pages

🧪 Testing

• Verified locally at http://localhost:3000
• Checked responsive design on mobile viewport
• Tested all code examples
• Validated all external links

📸 Screenshots (if applicable)

(Drag and drop ~/Desktop/dodo-docs-render.png here: the page rendered
through mintlify dev, showing the Affiliates nav group and the Steps setup.)

🔗 Related Issues

None.

📝 Additional Notes

The code examples follow the documented SDK shapes (client.payments.create
with product_cart and metadata, per your Dub guide's pattern and the
OpenAPI spec). Translated language folders are untouched per AGENTS.md; the
page is registered in the en navigation only.

One small heads-up from writing this: a few existing guides link
/api-reference/introduction#api-key-generation, but that heading is now
"API Key Management and Authentication", so those anchors no longer land. My
page links the current anchor; happy to fix the others in a follow-up if
useful.

👥 Reviewers

@dodopayments/documentation-team