Update README to similar format as in Terraform Foundation repository

[DanielCMiranda]

Updates in the README file:

• Change the README format to be similar to the one found in terraform-example-foundation repository
• Add Cloudbuild URL and Cloud Source Repositories URL as reference for the user
• Change the "sed" command into a cleaner format, separating into variables the information the user needs to change
• Fix repository names in some steps: 4-projects -> gcp-projects and 5-infrastructure -> boa-infra

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[DanielCMiranda]

@googlebot I signed it!

[daniel-cit]

/cc @bharathkkb @rutalreja-deloitte