docs: migrate URL

[erinecon]

What this PR does

This PR migrates the RTD documentation under the Canonical domain.

Why we need it

URL migrations are a high priority this cycle that we need to complete ASAP.

Checklist

• I followed the contributing guide
• I added or updated the documentation (if applicable)
• I updated docs/changelog.md with user-relevant changes
• [N/A] I added a change artifact for user-relevant changes in docs/release-notes/artifacts. If no change artifact is necessary, I tagged the PR with the label no-release-note.
• I used AI to assist with preparing this PR
• [N/A] I added or updated tests as needed (unit and integration)
• [N/A] If integration test modules are used: I updated the workflow configuration
  (e.g., in .github/workflows/integration_tests.yaml, ensure the modules list is correct)
• [N/A] If this PR involves Terraform: terraform fmt passes and tflint reports no errors
• [N/A] If this PR involves Rockcraft: I updated the version