Cleanup the Windows instructions for using conda/pixi.

[clalancette]

This makes the instructions match what CI does in ros2/ci#802 . This relies on ros2/ros2#1642 being merged first.

[github-actions]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/13164943814/artifacts/2543122460.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-4989/index.html in your favorite browser

[j-rivero]

Nice reduction in the number of steps required.

There are some references to chocolatey in the Troubleshooting and Contributing documents:

source/How-To-Guides/Installation-Troubleshooting.rst:We've seen that sometimes the chocolatey packages for ``asio``, ``tinyxml2``, etc. do not add important registry entries and CMake will be unable to find them when building ROS 2.
source/How-To-Guides/Installation-Troubleshooting.rst:We've not yet been able to identify the root cause, but uninstalling the chocolatey packages (with ``-n`` if the uninstall fails the first time), and then reinstalling them will fix the issue.
source/How-To-Guides/Installation-Troubleshooting.rst:- ``choco uninstall patch; colcon build --cmake-clean-cache`` - This is a bug in the `GNU Patch For Windows package <https://chocolatey.org/packages/patch>`_. If this package is not installed, the build process will instead use the version of Patch distributed with git.
source/The-ROS2-Project/Contributing/Windows-Tips-and-Tricks.rst:Unfortunately, the ``patch`` executable as delivered by chocolatey requires Administrator access to run.

Do we need to remove these ones?

[ros-discourse]

This pull request has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/upcoming-switch-of-windows-installation-to-pixi-conda/41916/1