Update installation and authentication docs

[geriom]

Changes (WIP)

• Add instructions to authenticate to Artifact Registry when running a Pipeline.

• Refactor Pipelines installation documentation

These documents reference each other.

This PR is just to generate the preview, not to be merged. The final update will comprise PRs to both the Website and the Pipelines repos.

• New Pipelines Installation docs
• New GCP authentication docs in how-to

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you review them:

• [ ] Includes docs (if user facing)
• [ ] Commit messages follow commit message best practices

See the contribution guide for more details.

[tekton-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[tekton-robot]

@geriom: PR needs rebase.

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