Docs: Add binascii.a2b_base64() entry

[swayam-mishra]

Description

This pull request introduces a new term entry for the binascii.a2b_base64() function in Python, as part of a Hacktoberfest contribution.

The new file is located at content/python/concepts/binascii-module/terms/a2b-base64/a2b-base64.md. It includes a clear definition, syntax, a standard example with output, and a runnable Codebyte example, all structured according to the Codecademy Docs term entry template.

Issue Solved

Closes #7696

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 review! 🚀