chore: [release-3.0.x] docs: Replace instances of k6.io/docs with grafana.com/docs/k6

[grafanabot]

Backport 57fde2d2aa88a313fd9b63d502eb0be2371def72 from #13302

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What this PR does / why we need it:

This PR updates all the links to k6.io/docs to grafana.com/docs/k6. This is part of the SEO work we're doing to try to make the Grafana version of the k6 docs to start to show up higher in search results.

I found these links by searching the website repository. Ideally, we'd update these links for all versions (v1.4.x to v1.7.x, and v2.5.x to v3.0.x).

Which issue(s) this PR fixes: Fixes #

Special notes for your reviewer:

Checklist

• [x] Reviewed the CONTRIBUTING.md guide (required)
• [ ] Documentation added
• [ ] 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.
• [ ] 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