feat(doc): scaffold draft for Track 3 release

[sed-i]

This PR updates a few files, scaffolding towards track 3 release.

Issue

Solution

This PR scaffolds the Track 3 release documentation by updating release notes, release policy, and several how-to/tutorial pages.

• Added a note in docs/reference/release-notes.md clarifying that COS track 3 is a release track for the COS bundle and does not correspond to any individual charm track on Charmhub.
• Updated stale/redirected links to their redirect targets across multiple documentation files:
  • docs/explanation/telemetry-labels.md: Updated Grafana blog relabeling URL
  • docs/how-to/selectively-drop-telemetry-scrape-config.md: Updated Grafana blog relabeling URL
  • docs/how-to/troubleshooting.md: Updated three Grafana docs URLs (dashboards/variables, add-template-variables, panel-editor-overview)
  • docs/tutorial/instrument-machine-charms.md: Updated juju.is → canonical.com/juju
  • docs/index.md: Updated the code-of-conduct link path

Checklist

• I have added or updated relevant documentation.
• PR title makes an appropriate release note and follows conventional commits syntax.
• Merge target is the correct branch, and relevant tandem backport PRs opened.

Context

COS track 3 is a versioning concept for the COS bundle itself; it does not map to a corresponding track on individual charm pages on Charmhub.

Testing Instructions

Review the updated documentation pages to verify links resolve correctly and the release notes accurately reflect the track 3 release scope.

Upgrade Notes

---

⚡ Quickly spin up Copilot coding agent tasks from anywhere on your macOS or Windows machine with Raycast.

[lucabello]

We should remind people that a track 3 won't reflect to any charm track

[sed-i]

@copilot update the following links so they point to the redirect target

(how-to/integrating-cos-lite-with-uncharmed-applications: line  121) redirect  https://documentation.ubuntu.com/server/how-to/security/install-a-root-ca-certificate-in-the-trust-store/index.html - with Found to https://ubuntu.com/server/docs/how-to/security/install-a-root-ca-certificate-in-the-trust-store/
(explanation/telemetry-labels: line  159) redirect  https://grafana.com/blog/2022/03/21/how-relabeling-in-prometheus-works - permanently to https://grafana.com/blog/how-relabeling-in-prometheus-works/
(how-to/troubleshooting: line  389) redirect  https://grafana.com/docs/grafana/latest/dashboards/variables/ - permanently to https://grafana.com/docs/grafana/latest/visualizations/dashboards/variables/
(how-to/troubleshooting: line  399) redirect  https://grafana.com/docs/grafana/latest/panels-visualizations/panel-editor-overview/ - permanently to https://grafana.com/docs/grafana/latest/visualizations/panels-visualizations/panel-editor-overview/
(how-to/troubleshooting: line  392) redirect  https://grafana.com/docs/grafana/latest/dashboards/variables/add-template-variables/#add-a-query-variable - permanently to https://grafana.com/docs/grafana/latest/visualizations/dashboards/variables/add-template-variables/
(how-to/selectively-drop-telemetry-scrape-config: line   65) redirect  https://grafana.com/blog/2022/03/21/how-relabeling-in-prometheus-works/ - permanently to https://grafana.com/blog/how-relabeling-in-prometheus-works/
(tutorial/instrument-machine-charms: line   18) redirect  https://juju.is - with Found to https://canonical.com/juju
(           index: line   81) redirect  https://ubuntu.com/community/ethos/code-of-conduct - with Found to https://ubuntu.com/community/docs/ethos/code-of-conduct

[Copilot]

We should remind people that a track 3 won't reflect to any charm track

Added a note in the release notes clarifying that COS track 3 is a release track for the COS bundle and does not correspond to any individual charm track on Charmhub (commit 545725e).

@copilot update the following links so they point to the redirect target

...

Updated all the listed links to point to their redirect targets in commit 545725e:

• explanation/telemetry-labels.md and how-to/selectively-drop-telemetry-scrape-config.md: grafana.com blog relabeling URL
• how-to/troubleshooting.md: three grafana.com docs URLs (dashboards/variables, add-template-variables, panel-editor-overview)
• tutorial/instrument-machine-charms.md: juju.is → canonical.com/juju
• docs/index.md: ubuntu.com/community/ethos/code-of-conduct → ubuntu.com/community/docs/ethos/code-of-conduct