docs: update contributing guide to manage python env with hatch shell

[wzxxing]

Description

Since the project is already using hatch as the build backend, it would be a more consistent development experience to also manage the python virtual environments using hatch shell.

I added a new env named dev so that contributor can use hatch shell dev to enter this virtual environment. It includes every optional features.

Type of Change

• Other (please describe): contributing guide

Testing

[How have you tested the change?]

yes

• hatch fmt --linter
• hatch fmt --formatter
• hatch test --all
• Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

Checklist

• I have read the CONTRIBUTING document
• I have added tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature
• My changes generate no new warnings
• Any dependent changes have been merged and published

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[Unshure]

Thanks for the contribution! Looks like the workflows are currently broken, but ive pulled this code locally and run it. Looks like everything passes!