Document Effect schema and HttpApiClient plugins

[nmnmcc]

🎯 Changes

• document @kubb/plugin-effect, including Effect v4 beta compatibility, schema output, DateTime.Utc, JSON int64 handling, and every public option
• document @kubb/plugin-effect-httpapiclient, including generated contracts and clients, authentication behavior, supported content types, constraints, and every public option
• add registry frontmatter so both plugins can be published on kubb.dev

Companion implementation: kubb-labs/plugins#696

✅ Checklist

• I have followed the steps in the Contributing guide.
• I am using USA English (serialize, color, center, canceled, analyze, license).
• I ran the humanizer check on any new documentation I wrote.

Validation: git diff --check origin/main...HEAD

This PR is intentionally a draft pending maintainer review.

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Latest commit: f2bc0c9

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