[Term Entry] C++ Math Functions: `isnan()`

[Dusch4593]

Reason/inspiration (optional)

Under cpp/concepts/math-functions/terms/, create a new term entry for the isnan() function.

• In the terms/ folder, make a new folder called isnan
• Inside the isnan folder, create a new markdown file called isnan.md
• Inside the file, be sure to include a "Syntax" section with a ```pseudo code snippet.
• Let's include an "Example" section with a ```cpp code snippet.
• Lastly, let's include a "Codebyte Example" with a runnable ```codebyte/cpp

Please refer to the entry templates, content standards, and markdown style guide when working on the PR for this issue.

Entry Type (select all that apply)

New Term for Existing Concept Entry

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