Commit terraform-docs changes if branch is from Renovate

[Stretch96]

• If Renovate makes changes to Terraform dependencies, the terraform docs workflow fails because the documentation hasn't been updated.
• This adds a new step, to run terraform-docs and push the changes.
• This also reverts the attempt to get Renovate to run a command after it has made it's updates.

[rjw1]

Do we just want to automate all the terraform docs stuff? Why is a human even doing it?

[Stretch96]

It made for a cleaner git history, because otherwise it adds in a load of 'Terraform Docs' commit messages.
They do disappear when using Squash and Merge, but it requires someone to remember to do that.