Move CONTRIBUTING.rst to the monorepo root.

### Summary of the feature request

Consolidate package level `CONTRIBUTING.rst` file contents into a single global file at the monorepo root level to expand upon/enhance the existing file.

**Current State**: 21 packages have an individual `CONTRIBUTING.rst` file.
**Similarity Profile**: 87.3% average similarity. Several packages are 99.8% identical (differing only by package name or tiny directory references).
**Why it's a prime candidate**: There is no need to maintain 21 versions of contributing guidelines. It creates severe maintenance overhead when guidelines or links change.

**Recommendation**:
Create a single master CONTRIBUTING.md or CONTRIBUTING.rst in the project root.
Delete the per-package CONTRIBUTING.rst files, or replace them with a minimal 2-line file pointing readers to the root guide:

> Looking to contribute?
> Please refer to the root `CONTRIBUTING.rst <../../CONTRIBUTING.rst>`_ for detailed guidelines.