docs: add prose documentation for core features to meet conformance

[DaleSeo]

Closes #692

Motivation and Context

Add prose documentation for 8 core MCP features to meet the Tier 2 SDK conformance requirement. All 8 features are already fully implemented but lack prose documentation. This PR adds a single docs/FEATURES.md covering each feature with server-side and client-side code examples, notification patterns, and links to runnable examples and the MCP spec.

Features documented: Resources, Prompts, Sampling, Roots, Logging, Completions, Notifications, Subscriptions.

Each section follows a consistent structure: overview, server-side implementation, client-side usage, notifications/change handling, and links to examples and spec. The approach is a hybrid of patterns from existing Tier 1 SDKs:

• TypeScript SDK (docs/server.md) - single-file structure, short descriptions with code examples and links
• Python SDK (README.md) - inline documentation with decorator-based examples
• C# SDK (docs/concepts/<feature>/) - most thorough, with separate server/client coverage and change notification patterns

How Has This Been Tested?

• All code examples are derived from existing, working examples in the repo
• All file links verified to point to existing files
• All type names, method signatures, and capability builder methods verified against the source code

Breaking Changes

None. Documentation only.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context