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	hide_classify: "OUTDATED"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.