Move CI/CD to GitHub Actions

[36degrees]

What

Move Frontend Docs' CI/CD from Travis to GitHub Actions.

Why

Travis' changes to our plan means that builds are getting stuck in a queue, which means they can take around 30 minutes to run.

Given that the direction of travel for the organisation seems to be towards GitHub Actions, and to be consistent with the approach we're taking with the Prototype Kit, move to GitHub Actions.

Who needs to know about this

Hanna, Laurence, Eoin

Further detail

Travis currently (as per the config) does two things:

• Builds the site and runs a link checker
• On the master branch, if the site builds and link checker passes, deploys the site to PaaS

Equivalent jobs need to be run using GitHub Actions.

There's an example of a workflow that deploys to PaaS from National Lottery Heritage Fund here:
https://github.com/heritagefund/funding-frontend/blob/master/.github/workflows/deploy-to-production-workflow.yml

We need to make sure that the production site can still be indexed, as the check is currently based on the TRAVIS_BRANCH environment variable.

We should make sure that the secrets are stored at the environment level with branch protection, as per https://github.com/alphagov/design-system-team-internal/issues/446.

Done when

• Use of secrets signed off with IA
• Existing open PRs merged / closed where possible to avoid needing to rebase
• GitHub Actions config added
• GitHub secrets added at environment level with branch protection
• Travis config removed
• noindex meta tag condition updated
• Required status checks updated
• Updated CODEOWNERS file
• Documentation updated:

govuk-frontend-docs/.rubocop.yml

Line 5 in 655c6e2

# Exclude `/vendor` because on Travis other gems get installed there

govuk-frontend-docs/config/tech-docs.yml

Lines 35 to 36 in d62c8d3

	# Set dynamically in config.rb depending on TRAVIS_BRANCH
	# prevent_indexing: false