docs: consolidate Training Tutorials and Evaluation Tutorials under a single Tutorials tab

[sephmard]

Part of #1436 — v0.4.0 IA re-org.

Currently Training Tutorials and Evaluation Tutorials are two separate top-level tabs. This ticket consolidates them under a single Tutorials tab with two collapsible sections, keeping all existing page groupings intact.

Proposed nav structure

Tutorials
├── Training Tutorials        ← section heading / dropdown
│   ├── NeMo RL (GRPO)       ← existing sub-pages unchanged
│   │   ├── About the Workplace Assistant
│   │   ├── Setup
│   │   ├── Gym Configuration
│   │   ├── NeMo RL Configuration
│   │   ├── Single-Node Training
│   │   └── Multi-Node Training
│   ├── Unsloth
│   ├── Multi-Environment Training
│   ├── Training with VeRL
│   └── Offline Training (SFT/DPO)
└── Evaluation Tutorials      ← section heading / dropdown
    ├── Evaluate EvalPlus
    └── BLADE Analysis Skill

Tasks

• Update fern/versions/main.yml to replace the two separate folder entries for training-tutorials and evaluation-tutorials with a single section: Tutorials block using the Fern section + contents syntax:

  - section: Tutorials
    contents:
      - folder: ./latest/pages/training-tutorials
        title: "Training Tutorials"
        title-source: frontmatter
      - folder: ./latest/pages/evaluation-tutorials
        title: "Evaluation Tutorials"
        title-source: frontmatter

• Verify that page URLs are unchanged (folder paths stay the same — no redirects needed)
• Add redirect in fern/docs.yml if the section slug differs from the previous top-level slugs (check Fern behavior for section vs folder slug generation)
• fern check passes with 0 errors

Note

Folder paths (training-tutorials/, evaluation-tutorials/) do not change, so existing URLs remain stable. Only the nav grouping changes.