docs: Update _index.md: add information about non-blocking mode to the documentation (backport release-3.2.x)

[loki-gh-app]

Backport 10fef0c from #15910

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What this PR does / why we need it:

I struggled with the issue that after working for more than 10 minutes without an internet connection, all my containers froze and stopped working. I've spent a lot of time fixing this issue and I couldn't even find any mention of such a parameter in the documentation.
Thanks for https://community.grafana.com/t/loki-driver-on-docker-and-getting-stuck-container/56610/2

Which issue(s) this PR fixes:

If the Loki server is unavailable, the running containers run for 10 minutes and freeze. That is, everything that is running inside the container completely stops working, which is unacceptable in my opinion. I was also confused that the documentation doesn't even mention that there is such a log-opt!

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR