docs: use an executable path in the try_first_with example

[travisfortney]

Hi! I'm getting some practice with small GitHub contributions and noticed the standalone guide passes a directory to try_first_with.

That argument probes interpreter executables directly. Passing a directory raises a permission error, so this updates the wording and example to use /opt/python/bin/python3.12, plus a documentation news fragment.

Checked locally on macOS:

• Reproduced the directory failure and successful executable selection against current source.
• All 13 default tox environments, plus dev: Python 3.9–3.15 and free-threaded 3.15, both type targets, lint, docs and package checks.
• Fresh strict documentation build, rendered example/news inspection and the staged Towncrier check.

Native Linux/Windows and legacy-target jobs were not run locally.

Thanks for taking a look!