[Concept Entry] C++ Deque Deque

[codecademy-docs]

Reason/inspiration (optional)

"We would like a new concept entry for C++ on Deque. Provide the entry in a markdown file under docs/content/cpp/concepts/deque/deque.md

The entry should - Have an introductory paragraph about the deque data structure. - A syntax section with a pseudo code block with an explanation of it. - An example showing the use of deque. - Explain the equivalent method for stacks and queue. Please refer to the concept entry templates, content standards, and markdown style guide when working on the PR for this issue."

Entry Type (select all that apply)

New Concept Entrty for Existing Topic

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