docs: add Pandas groupBy method .agg() term entry #7690

[vansh2408]

Description

This PR adds a new term entry for the .agg() method under groupBy in Pandas, following the Codecademy Docs standards and structure.

Issue Solved

Closes #7690

Type of Change

• Adding a new entry

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.

[CLAassistant]

All committers have signed the CLA.

[mamtawardhani]

Looks good for a second round of review! 🚀

[dakshdeepHERE]

@vansh2408 If you want me to proceed with 2nd review please sign the CLA agreement.

[vansh2408]

@vansh2408 If you want me to proceed with 2nd review please sign the CLA agreement.

Done.

[dakshdeepHERE]

Entry looks good for merging 👍

[github-actions]

👋 @vansh2408
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