Docs/improve contributing guidelines

[paularamo]

Summary

• Replaces the generic CONTRIBUTING.md with a structured contributing guide that establishes clear quality requirements for community-contributed applications, provides a ready-to-copy README template, and defines the cookbooks/cosmos3/applications/ directory as the home for external recipes.
• Adds the cookbooks/cosmos3/applications/ directory with reasoner/ and generator/ subdirectories (audiovisual, action, transfer), creating a clean contribution surface for the community without modifying the existing NVIDIA-shipped cookbook structure.
• Adds an Applications section to the main README.md Table of Contents and body, surfacing the new contribution path for developers.

Contributing Guide Enhancement

The current CONTRIBUTING.md is a 50-line generic fork-commit-PR boilerplate with no guidance on application structure, quality bar, or placement. This PR replaces it with a production-grade contributing guide that includes:

• Applications directory structure — Documents the cookbooks/cosmos3/applications/ tree with a routing table so contributors know exactly where their content belongs (reasoner, generator/audiovisual, generator/action, generator/transfer).
• Six quality requirements reviewers check on every PR:
  1. Open-access data only (no internal S3, VPN-only URLs, or proprietary datasets)
  2. Results / Expected Output section (sample outputs, timing benchmarks, loss curves)
  3. Canonical setup (link to shared README, no duplicated install steps)
  4. One-click runnable (notebooks run top-to-bottom without manual intervention)
  5. Naming convention (run_<task>_with_<backend>.ipynb)
  6. Author attribution (author name and organization in README and notebook)
• Application README template — Ready-to-copy structure with title, author block, backends table, quick start, results section, and dataset table with license.
• Contribution areas table — Clear "what we accept" and "what we won't merge" boundaries.
• Testing checklist — Clean kernel run, minimal GPU, no secrets, file size limits.

Applications Directory

This PR adds cookbooks/cosmos3/applications/ as the designated home for community and external contributions. The directory mirrors the tower structure used by the core cookbooks:

cookbooks/cosmos3/applications/
├── README.md
├── reasoner/
│   └── README.md
└── generator/
    ├── README.md
    ├── audiovisual/README.md
    ├── action/README.md
    └── transfer/README.md

No existing files are moved or renamed — this is a pure addition that creates the contribution surface alongside the existing NVIDIA-shipped cookbooks.

Test Plan

• Verify the CONTRIBUTING.md directory tree matches the new applications/ structure
• Confirm the Application README template renders correctly on GitHub
• Check that the main README.md Applications section links resolve correctly
• Verify each applications/ subdirectory README links back to the contributing guide

[nv-asotelo]

LGTM - Need repo owners to weigh in on the contributing guidelines and migration of the basic examples to the respective directories.

[lfengad]

LGTM

[foreverlms]

@paularamo Hi, Sorry that we get this PR delayed. Now the main branch naming rules has changed, could you please resolve these conflicts and we could merge this asap?

[paularamo]

@foreverlms the conflict has been solved.

[lfengad]

@yaoxu-crypto Please have a review of this?

[yaoxu-crypto]

Awaiting resolution on this: https://nvidia.slack.com/archives/C0AQFL327FH/p1781556961949859

[yaoxu-crypto]

• The file paths should be like this: cosmos/cookbooks/cosmos3/generator/action/finetune
• does this guideline include file naming convention for the recipes?