Enable gh pages deployment for gh Actions-based workflow

[milesfrain]

I was hoping that merging #343 or #344 would deploy the latest version of the book to gh pages, but it's skipping deployment with this error:

This action runs on a fork and not found auth token, Skip deployment

https://github.com/purescript-contrib/purescript-book/actions/runs/854860824

Here's the relevant code:

purescript-book/.github/workflows/mdbook.yml

Lines 21 to 27 in 6dacfa9

	- name: Deploy
	uses: peaceiris/actions-gh-pages@v3
	if: github.ref == 'refs/heads/master'
	with:
	deploy_key: ${{ secrets.GITHUB_REPO_DEPLOY_KEY }}
	publish_dir: ./book
	cname: book.purescript.org

Thinking it could either be due to an incompatibility of github.ref == 'refs/heads/master' with this repo being a fork of paf31's version, or that GITHUB_REPO_DEPLOY_KEY is unavailable.

Documentation for this action is available in https://github.com/peaceiris/actions-gh-pages#options

@hdgarrood , we worked through this before in #82. I was hoping that GITHUB_REPO_DEPLOY_KEY would still be available for use, but can't inspect the repo's secrets myself to double-check. Could you take a look at this?

[milesfrain]

Found some more evidence that this is an issue with an empty GITHUB_REPO_DEPLOY_KEY (assigned to deploy_key). It should work on forks if deploy_key is non-empty.

This version can skip deployment on forks when deploy_key or personal_token is empty.

peaceiris/actions-gh-pages#153 (comment)

[hdgarrood]

There are no secrets configured for this repo right now; I guess I removed the deploy key after we ended up not using it for Travis. I can try adding one again shortly.

[hdgarrood]

According to https://github.com/peaceiris/actions-gh-pages, GitHub automatically creates a token called GITHUB_TOKEN and this doesn't require any setup. That seems ideal. Could you try doing it that way?

[milesfrain]

Fixed by #346