CI: Per-PR doc previews are not always cleaned up

[leofang]

Compare the number of folders in the gh-pages branch
https://github.com/NVIDIA/cuda-python/tree/gh-pages/docs/pr-preview
with the number of open PRs that we have:
https://github.com/NVIDIA/cuda-python/pulls
I think the numbers mismatch.

Some logic was oversimplified in the doc_preview action, for example:

cuda-python/.github/actions/doc_preview/action.yml

Lines 47 to 69 in 99ffe14

	# The steps below are executed only when building on main.
	- name: Remove doc preview
	if: ${{ github.ref_name == 'main' }}
	uses: JamesIves/github-pages-deploy-action@v4
	with:
	git-config-name: cuda-python-bot
	git-config-email: cuda-python-bot@users.noreply.github.com
	folder: ${{ inputs.source-folder }}
	target-folder: docs/pr-preview/pr-${{ inputs.pr-number }}/
	commit-message: "Clean up doc preview for PR ${{ inputs.pr-number }} (${{ github.sha }})"
	- name: Leave a comment after removal
	if: ${{ github.ref_name == 'main' }}
	uses: marocchino/sticky-pull-request-comment@v2
	with:
	header: pr-preview
	number: ${{ inputs.pr-number }}
	hide_and_recreate: true
	hide_classify: "OUTDATED"
	message: |
	Doc Preview CI
	:---:
	Preview removed because the pull request was closed or merged.

These steps are only run when the target branch is main. So one class of cleanup failures came from the previous path finder work, where PRs targeted a feature branch instead of the main branch. However, this bug alone does not seem to account for the discrepancy fully; there seem to exist other issues that I could not tell immediately.

[leofang]

Another class of errors: The doc job is canceled (due to multiple merges), e.g. b9427f1

[leofang]

In the gh-pages branch, under the folder docs/pr-preview, there are subfolders pr-XXXXX created to host doc preview for the corresponding PR. We need a bash script placed in the main branch, under the ci folder, that runs through all preview pr-XXXXX folders, check if the corresponding PR XXXXX is already merged or closed, and if so remove the pr-XXXXX folder. Keep the bash script runnable manually for now. We can consider adding a CI workflow to run it periodically later.