Add example listening for sigterm to docs (APMSP-15340)

[ajgajg1134]

What does this PR do?

Add an example so customers know they can listen for SIGTERM and shutdown the tracer to avoid lost data.
I debated adding more complexity here like enforcing a timeout, or supporting a context cancellation so this go routine doesn't just hang around forever but that seems like something that "can be left as an exercise to the reader" perhaps? If others feel strongly I'm happy to update this example to be more in depth though.

Motivation

See #2976 for some motivation, at some point we may attach this sort of listener automatically but there doesn't seem to be a clean way to do so currently without risking altering some customer application behavior.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-11-13 20:08:55

Comparing candidate commit 6895d33 in PR branch andrew.glaude/sigNotifyDocs with baseline commit f28ac98 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 58 metrics, 1 unstable metrics.