Create GitHub action that cleans up the Docs preview versions from PRs

[navidcy]

based on https://github.com/CliMA/TimeMachine.jl/blob/master/.github/workflows/DocCleanup.yml

[charleskawczynski]

The GHA in https://github.com/CliMA/TimeMachine.jl/blob/master/.github/workflows/DocCleanup.yml seems to look a bit different. In particular, the if [ -d "previews/PR$PRNUM" ]; then in TimeMachine.jl seems like this happens conditionally, which doesn't seem to be the case in this PR. Is this an updated recommended version from somewhere?

[navidcy]

The GHA in https://github.com/CliMA/TimeMachine.jl/blob/master/.github/workflows/DocCleanup.yml seems to look a bit different. In particular, the if [ -d "previews/PR$PRNUM" ]; then in TimeMachine.jl seems like this happens conditionally, which doesn't seem to be the case in this PR. Is this an updated recommended version from somewhere?

I got this from Documenter.jl Docs --> https://juliadocs.github.io/Documenter.jl/stable/man/hosting/#gh-pages-Branch

[navidcy]

bors r+

[bors]

Build succeeded:

• docs-build
• format
• test_success