Fix terraform-docs workflow branch name check

[Stretch96]

• For PRs, github.ref returns the format refs/pull/{pull_request_number}/merge, which isn't what we want
• This fixes it by getting the branch name using ${GITHUB_REF#refs/heads/} in a previous step, then checking the name in the terraform docs steps

[DrizzlyOwl]

LGTM. Shame about the repetition. I wonder if we can extract the renovate specific ones into their own workflow so its separate to the linting tasks

[Stretch96]

Terraform docs relies on terraform init being ran, so I've put them in the same workflow
I'm not sure if we did run it in a separate workflow (which ran after the validate workflow), that it would use the same generated output from terraform init, or if we'd have to run that again