Update _index.md (#5934)
Update _index.md (#5934)

changed URLs for [Fluentd](https://github.com/grafana/loki/tree/main/clients/cmd/fluentd) and [Fluent Bit](https://github.com/grafana/loki/tree/main/clients/cmd/fluent-bit) since the old URLs where leading to a 404 on GitHub.
grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki