Docs: Pattern for maintaining versioned example code (maintaining latest versions of Python, Sphinx etc.)

[benjaoming]

In #9838, we updated examples of 2 .rst files to have the latest Python version available 👍

There are still some files left to look at:

docs/user/faq.rst
260:      - python=3.9

docs/user/tutorial/index.rst
325:       python: "3.8"
341:You will notice that one of the lines contains ``python3.8``:
345:.. figure:: /_static/images/tutorial/build-python3.8.png
388:       python: "3.8"

docs/user/environment-variables.rst
106:       python: 3.10

docs/user/guides/conda.rst
51:      - python=3.8
59:   whereas ``python=3.8`` (single ``=``) will fetch the latest ``3.8.*`` version,
98:      - python=3.8
157:      - python=3.8
194:      - python=3.8

...moreover, if we can find a way to maintain the current default Python version as |example_python_version| or similar (+ |example_sphinx_version| etc), that would be great. It's a bit risky since the context may be unknown. But looking at how many outdated examples there are, it might be preferable to manually maintaining this.

[humitos]

if we can find a way to maintain the current default Python version as |example_python_version| or similar (+ |example_sphinx_version| etc)

I think this is the way to go here. Keep the version to display to users in just one place and always use the alias everywhere 👍🏼