docs: Add an example app and/or docs article for test mode

[schloerke]

Follow-up to #2269 / PR #2270.

Test mode (the SHINY_TESTMODE=1 / App(test_mode=) flag, shiny.session.export_test_values(), the /dataobj/shinytest snapshot endpoint + session.get_test_snapshot_url(), and the shiny.playwright.controller.AppTestValues controller) currently only has API docstrings.

Add user-facing documentation:

• A shiny/api-examples/ example app demonstrating export_test_values() and reading the snapshot with AppTestValues in an end-to-end test, and/or
• A short docs article/how-to on testing a Shiny app's reactive state with test mode.