[Concept Entry] C++ Unordered Map

[codecademy-gh-api]

Reason/inspiration (optional)

We would like a new entry on the Unordered_Map in C++. The entry should go in a new file under docs/content/cpp/concepts/unordered-map/unordered-map.md.     

The entry should include:    

• An introduction to the concept    
• A Syntax section that provides the syntax for the concept (if applicable)    
• An Example section that provides an example demonstrating the concept in use (if applicable)   
• An Codebyte Example section that provides an example demonstrating how to use deque

Please refer to the concept entry template, 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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