docs: add `X-Scope-OrgID` header to Grafana datasource if multi-tenancy is enabled

[jonathanfoster]

What this PR does / why we need it:

This PR updates the Visualize Grafana doc to include instructions for adding Loki datasource when multi-tenancy is enabled.

Which issue(s) this PR fixes:

Fixes #13179. Well, the OP for this issue hasn't confirmed the fix, but I ran into a similar issue and did notice the docs were missing this key info. I'm happy to open a new issue if that makes more sense.

Special notes for your reviewer:

Checklist

• [X] Reviewed the CONTRIBUTING.md guide (required)
• [X] Documentation added
• [X] Tests updated
• [X] 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.
• [X] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• [X] 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
• [X] 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

[CLAassistant]

All committers have signed the CLA.

[grafanabot]

This PR must be merged before a backport PR will be created.

[grafanabot]

This PR must be merged before a backport PR will be created.

[loki-gh-app[bot]]

This PR must be merged before a backport PR will be created.