docs: add examples for local_tempdir() and with_tempdir()

[LeonidasZhak]

Closes #283

Adds runnable examples for local_tempdir() and with_tempdir() to the shared with_tempfile help page. Previously only local_tempfile() and with_tempfile() had examples.

Examples demonstrate:

• local_tempdir(): returns a path, auto-deletes on scope exit
• with_tempdir(): temporarily changes working directory, restores after

Validation:

• tools::checkRd() passes
• Examples run successfully
• R CMD check: pre-existing errors in with_locale and test-language.R (unrelated)

[LeonidasZhak]

Withdrawing this because the checks are failing and this small documentation PR is not worth adding review burden in its current state. Sorry for the noise, and thank you for maintaining the project.