[Term Entry] Python deque: index() (Codecademy#7755)

[LEpessoa]

• Adding index.md ---------

Description

Added a term entry for the index() method of the deque class in the documentation.

Issue Solved

#7755

Type of Change

• Adding a new entry
  - Editing an existing entry (fixing a typo, bug, issues, etc)
• 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.

[mamtawardhani]

Looks good for a second review! 🚀