Add documentation for Python package nbgv-python

[Copilot]

Documents the community-contributed Python package nbgv-python that adapts NBGV to the Python ecosystem.

Changes

• Added docfx/docs/ecosystems/python.md with package overview and installation instructions
• Updated docfx/docs/toc.yml to include Python in the Ecosystems navigation
• Updated docfx/docs/getting-started.md to list Python alongside .NET, Node, and VSIX ecosystems

Documentation is intentionally brief since the package is externally maintained—users are directed to PyPI for usage details.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Promption: nbgv for Python</issue_title>
<issue_description>I'd like to let you know that I wrote and published a Python package https://pypi.org/project/nbgv-python/ to adapt NBGV to python world.</issue_description>

<agent_instructions>Update documentation to reference this python package.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Promotion: nbgv for Python #1313

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