docs(cpp/deque): add .back() term entry

[HackersSpirit]

Summary: Add a new C++ term entry for deque::back().
Includes:
Description
Syntax (pseudo block + details)
Example (cpp) with Output (shell)
Codebyte Example (codebyte/cpp)
Notes:
First mention bolded
Added note on undefined behaviour for empty deque
Follows term-entry-template, content-standards, and style-guide

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ mamtawardhani
❌ HackersSpirit
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[mamtawardhani]

Hey @HackersSpirit, please sign the CLA

[mamtawardhani]

Looks good for a second review! 🚀

[mamtawardhani]

@HackersSpirit please sign the CLA otherwise we'll have to close this PR

[ylevental]

@mamtawardhani When will this be closed? It's been two weeks...

[mamtawardhani]

Hey @ylevental, the team is busy with loads of other work as well. We'll close and recycle the PRs as and when time permits. Hopefully by the end of october 😄

[mamtawardhani]

Closing this PR due to inactivity of the Author.