Update documentation to reflect stability #1836

Need: User and developer documentation should reflect that the Python Environments extension is stable and is the default environment experience. Experimental, rollout, and opt-in language will be misleading once the experiment is removed.

Acceptance criteria

• User-facing documentation describes the extension as stable and enabled by default.
• Obsolete experiment, rollout, opt-in, and opt-out guidance is removed or replaced.
• References to python.useEnvironmentsExtension are removed where the setting no longer applies.
• API documentation clearly identifies the supported stable API version.
• Troubleshooting guidance no longer directs users to switch back to the legacy environment experience.
• Links and examples are checked against the final post-experiment behavior.

This is a prerequisite for fully turning off the Python Environments experiment. Tracked by #1835.