docs(ci): clarify builder and credential variables for CI setup

[nico1510]

Description

This PR updates the Docker Build Cloud CI documentation to clarify the use of builder and credential variables.

Key changes:

• Standardizes the use of DOCKER_ACCOUNT, DOCKER_ACCESS_TOKEN, and CLOUD_BUILDER_NAME in all CI/CD examples.
• Updates template string syntax for builder endpoints (e.g., ${{ vars.DOCKER_ACCOUNT }}/${{ vars.CLOUD_BUILDER_NAME }} in GitHub Actions).
• Adds explicit call outs that the cloud builder has to be created first.
• Improves instructions for naming and referencing cloud builders in both the dashboard and CLI steps.
• Ensures all code comments and documentation are consistent and clear for users setting up Docker Build Cloud in CI.

These changes make it easier for users to configure their CI/CD pipelines correctly and reduce confusion around builder and credential setup.

Related issues or tickets

https://docker.atlassian.net/browse/DCL-248

Reviews

• [ ] Technical review
• [ ] Editorial review
• [ ] Product review

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[nico1510]

@fiam @silvin-lubecki could you PTAL?