Rename govuk-frontend-docs default branch to `main`

What

Rename the master branch to main.

Why

Many communities on GitHub are considering renaming the default branch name of their repository from master, as this terminology can be offensive. Statements and guidance have been published by Git here and GitHub here.

New GitHub repositories use main as the name for the default branch, so it makes sense that we should follow this convention, as well as matching what other teams within GDS are doing so we are consistent.

Additional Info

GitHub have made it easier for teams to rename branches. When we rename the branch, we get the following for "free" without having to do anything ourselves:

Open PRs and branch protection rules changed automatically
Web requests for the old branch name are redirected to the new branch name
API requests for the old branch name return a Moved Permanently response
A notice is shown on the repository homepage to notify contributors, maintainers and admins to update their local copies of the repo
A notice is shown to contributors who git push to the old branch

Who needs to know about this

Developers

Steps to follow

[x] Raise PR to switch functionality that relies on master and to update any documentation
[x] Once the PR has been approved (not merged!), rename the master branch to main in the Github UI
[x] Double check branch protection rules
[x] Double check webhooks
[x] Double check any deploy systems, e.g: Heroku, that may have it's own config
[x] If the repo uses Travis: Travis has its own idea of what the 'default branch' is which is synced once a day. You can manually trigger a sync by going to https://travis-ci.com/account/repositories and pressing 'sync account'.
[x] Merge the PR
[x] Check everything is still working as expected, e.g: build and deploy. If so, double check there is a main branch present and no old master branch present.

Done when

[x] The default branch is named main
[x] Update Travis config to deploy only when on the main branch
[x] Update indexing check to look for main branch
[x] Documentation has been updated to refer to main rather than master
[x] Check local setups still work
[x] Update the pinned issue on this repo saying the work has been completed
[x] Update the progress list in the govuk-design-system pinned issue

alphagov / govuk-frontend-docs