docs: add C++ fpclassify() documentation

[Priyanshjain10]

Added comprehensive documentation for the C++ fpclassify() math function.

Description

This PR adds a new term entry for fpclassify() under the C++ math-functions concept, providing complete documentation including syntax, parameters, return values, and examples.

What's Included

• Description and syntax details
• Return value explanations (FP_INFINITE, FP_NAN, FP_ZERO, FP_SUBNORMAL, FP_NORMAL)
• Working example with multiple test cases
• Interactive codebyte example for hands-on learning

Type of Change

• Adding a new entry

Checklist

• [x] All writings are my own
• [x] My entry follows the Codecademy Docs style guide
• [x] My changes generate no new warnings
• [x] I have performed a self-review of my own writing and code
• [x] I have checked my entry and corrected any misspellings
• [x] I have made corresponding changes to the documentation if needed
• [x] I have confirmed my changes are not being pushed from my forked main branch
• [x] I have confirmed that I'm pushing from a new branch named after the changes I'm making
• [x] I have linked any issues that are relevant to this PR in the "Issue Solved" section

Closes #7990