docs: update index.md

[ddevsr]

Description See https://queue.codeigniter.com/

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [x] User guide updated
• [x] Conforms to style guide

[kenjis]

Duplicate of #43