Fix #3905

Problem

When mkdocs serve receives SIGTERM (e.g., when a Docker container is stopped), the process is killed immediately without running cleanup code. The temporary site_dir created in /tmp is never deleted, causing directories to accumulate.

Only KeyboardInterrupt (SIGINT / Ctrl+C) was handled, which triggered the cleanup path.

Fix

Register a SIGTERM signal handler that raises KeyboardInterrupt. This triggers the existing cleanup path in the except KeyboardInterrupt / finally blocks, which calls server.shutdown() and shutil.rmtree(site_dir).

After the first SIGTERM is caught, the handler is restored to SIG_DFL so a second SIGTERM during cleanup will force-kill the process.

Verification

• All 725 existing tests pass (same pre-existing failures in localization tests)
• Source inspection confirms signal.signal(signal.SIGTERM, ...) is registered with the _sigterm_handler that raises KeyboardInterrupt