Create workflow to delete preview (folders) of closed PRs.

[NPDebs]

👍🎉 First of all, thanks for taking the time to contribute! 🎉👍

This PR follows on #948, where a workflow was set up to preview PRs before merging. Here, we are automatically deleting the PR-specific subfolder created when the PR is first deployed to gh-pages branch of ols-site-preview repo.

FOR CONTRIBUTOR

• I have read the CONTRIBUTING.md document

PR Type:

• This PR adds a new blog post
• This PR does something else (explain above)

FOR REVIEWERS

Thanks for taking the time to review! ❤️

Here are the list of things to make sure of:

• The website builds (a check will fail if not)
• All images have been added within the Pull Request and they have Alt text
• If there are paragraphs or text, the key messages are highlighted
• All internal links (within OLS website) use the {% link path_to_file.md %} format
• The preview corresponds to the changes described in the Pull Request
• The code is tidy and passes the linting tests