docs: add missing `.md`

[kenjis]

Description

INFO    -  Doc file 'index.md' contains an unrecognized relative link 'basic-usage', it was left as is. Did you mean 'basic-usage.md'?
INFO    -  Doc file 'index.md' contains an unrecognized relative link 'running-queues', it was left as is. Did you mean 'running-queues.md'?

Checklist:

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

[michalsn]

Thanks!