added deque and updated index.md

[VaishnaviMankala19]

PR Description

I have added deque : implementation , examples and applications. under ds-algorithms

Related Issues

• Closes #1244

Checklist

• [x] I have gone through the contributing guide
• [x] I have updated my branch and synced it with project main branch before making this PR

Undertaking

I declare that:

1. The content I am submitting is original and has not been plagiarized.
2. No portion of the work has been copied from any other source without proper attribution.
3. The work has been checked for plagiarism, and I assure its authenticity.

I understand that any violation of this undertaking may have legal consequences that I will bear and could result in the withdrawal of any recognition associated with the work.

• [x] I Agree

[ashitaprasad]

Spend few days writing the content properly before sending a PR.