docs: add missing examples for commitment-related methods

[thecaptain789]

Summary

This PR adds practical code examples for commitment-related methods in both subtensor.py and async_subtensor.py, addressing existing TODO comments in the codebase.

Changes

Added examples for the following methods:

get_all_commitments()

• Shows how to retrieve and process commitment data for all neurons in a subnet
• Demonstrates iterating through results and filtering active commitments

get_commitment()

• Demonstrates how to retrieve commitment data for a specific neuron by UID
• Shows proper handling of empty/missing commitment cases

get_commitment_metadata()

• Illustrates how to fetch and handle raw commitment metadata from the blockchain
• Shows type checking for the Union return type (dict | str)

Motivation

These methods had TODO comments requesting realistic examples:

• TODO add example of how to handle realistic commitment data
• TODO: add a real example of how to handle realistic commitment data, or chop example
• TODO: how to handle return data? need good example @roman

The examples follow the existing documentation style and provide practical guidance for developers using the commit-reveal mechanism.

Testing

Documentation-only change. Examples use standard SDK patterns and have been verified for correctness against the method signatures.