Remove venv from the Python preamble and adjust pip usage

[mmmarinho]

This PR removes all references to venv from the tutorial while keeping the Python preamble steps working.

Why

The preamble currently creates/activates a ros2tutorial_venv and routes user-level pip installs through it. This PR drops the venv entirely. Without it, user-level pip installs run against the system Python with --break-system-packages — which is consistent with the dqrobotics example already used in the same section and works on the target Ubuntu 24.04 (PEP 668).

What changed (6 files, docs only)

• installing_python.rst — removed the "When you want to isolate your environment, use venv" section (create/activate/deactivate) and the python3-venv apt install; adjusted the dqrobotics example to drop the activate step.
• python_best_practices.rst, python_asyncio.rst, python_packaging.rst — removed the "Use a venv" sections and the source ros2tutorial_venv/bin/activate commands.
• python_packaging.rst — added --break-system-packages to the pip install/uninstall commands that previously relied on the venv, and updated the sample output paths to the user site-packages location.
• the_canonical_build_command.rst — removed the "colcon will not work with an active venv" warning. (This snippet is .. include::d by ~14 pages, so the warning disappears from all of them.)
• source_after_build.rst — rewrote the "dirty state" fix to drop the venv framing while keeping the useful build/ install/ log/ cleanup advice.

Verification

• Full Sphinx build still succeeds — warning count is unchanged vs. main (no new warnings, no removed warnings; the only diff is a line-number shift on a pre-existing, unrelated warning).
• Grep sweep confirms zero remaining venv / virtual environment / ros2tutorial_venv / activate / deactivate mentions in the docs.
• No dangling cross-references (the removed Isolate your environment with a venv anchor was only referenced by the three sections that were also removed).

Note to maintainer: the only behavioral change is that the sample packages (wheel, minimalist_package) now install to the user's real site-packages instead of a throwaway venv — same trade-off the tutorial already makes with dqrobotics.

[mmmarinho]

Trying out this year without venv as it was a constant source of confusion.