[docs] fill missing c++ logs documentation

[hxiao87]

• I have read and followed the Contributing docs, especially the "First-time contributing?" section.
• This PR has content that I did not fully write myself.
  • I used AI and I have read and followed the Generative AI Contribution Policy.
• I have the experience and knowledge necessary to understand, review, and validate all content in this PR.¹

---

changes

This change is to address issue #9548, by replacing the placeholder "documentation missing" notice in the Logs section for en/cpp with actual content covering:

• exporter and processor setup
• logger provider registration
• emitting log record

The code examples in the change are adapted from this example.

Footnotes

1. Yes, I can answer maintainer questions about the content of this PR, without using AI. ↩

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: hxiao87 / name: Han Xiao (0c561a6, 41fe3e0, 818174f, 881d39a, f03f72d)
• ✅ login: hxiao87 / name: hxiao87 (16e41da, 69577fa, a33d4a3)
• ✅ login: otelbot[bot] / name: otelbot (81f105e, 9108979)
• ✅ login: vitorvasc / name: Vitor Vasconcellos (db5e44f, 16e41da)

[vitorvasc]

/fix:refcache

[otelbot-docs]

✅ fix:refcache applied successfully in run 24679635721.

[hxiao87]

/fix:refcache

[otelbot-docs]

✅ fix:refcache applied successfully in run 24753493274.

[svrnm]

@open-telemetry/cpp-approvers PTAL!

[dbarker]

Thanks for the contribution. Please see some optional points for improvement below.

[otelbot]

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