[release-2.9.x] docs: Update _index.md addressing #11575

[grafanabot]

Backport 3b2278d from #11626

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What this PR does / why we need it:
It improves the docs by addressing #11575 allowing users to use the official fluent-bit helm chart over the deprecated grafana one

Which issue(s) this PR fixes:
Fixes #11575

Special notes for your reviewer:
@JStickler wanted to take a look at this.

Checklist

• [ x ] Reviewed the CONTRIBUTING.md guide (required)
• [ x ] Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• 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

[github-actions]

🚫 [doc-validator] _{reported by reviewdog 🐶}
The link target 'https://grafana.com/docs/loki/latest/setup/install/helm/' is invalid. Links to grafana.com pages must not include the hostname so that they can be resolved regardless of domain.
Links to grafana.com pages must be one of the following:

• For linking to headings within the current page, use an anchor starting with a hash (#).
  For example, #heading.

• For linking to other pages in the current documentation set, use a Hugo relref with either an absolute path or relative path parameter.
  Using a relative path parameter, for example, {{< relref "./path/to/page" >}} or {{ relref "../other/path" }}.
  Using an absolute path parameter, for example, {{< relref "/docs/grafana-cloud" >}}.

• For linking to any other page in the https://grafana.com site, use a partial URI consisting of an absolute path.
  For example, /blogs/.

For an explanation of partial URIs, refer to https://www.w3.org/Addressing/URL/4_3_Partial.html.