[Edit] Combine Duplicate Focus Group Entries

[ebikatsudon]

Description

Combined and edited duplicate focus-group.md and focus-groups.md entries into one file.

Issue Solved

Closes #5069

Type of Change

• Editing an existing entry
• Updating the documentation

Checklist

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