docs: Add C++ unordered_set::cbegin() term entry (#8031)

[Rishabh1925]

Closes #8031

This PR adds the documentation entry for the C++ unordered_set::cbegin() method.

Implementation Details:

• File Path: Created the file at the required location: docs/content/cpp/concepts/unordered-set/terms/cbegin/cbegin.md.
• Core Concept: The entry clarifies the use of the constant iterator (const_iterator) returned by cbegin() for safe, read-only iteration over the set's elements.
• Structure: Includes the mandatory Description, Syntax, and Example sections, along with the required Codebyte.

[Rishabh1925]

Hello @mamtawardhani @codecademy-docs

I've finished creating the new term entry for unordered_set::cbegin(). The documentation highlights the use of the constant iterator for read-only access. Ready for review!

[mamtawardhani]

Looks good for a second review! 🚀