CI is still skipped when there are non-doc modifications

[strongoier]

https://github.com/taichi-dev/taichi/runs/4252103913?check_suite_focus=true is an example run. Although docs/lang/articles/basic/type.md is modified, there are other non-doc modifications. However, CI is skipped in this case.

The logic here:

taichi/.github/workflows/presubmit.yml

Lines 29 to 32 in 17a0a11

	if [[ $file == docs/* ]]; then
	echo "This modified file is not under the 'docs' folder."
	echo "::set-output name=run_job::false"
	break

is incorrect. We can skip CI only if all modifications are within docs.

[strongoier]

Fixed by #3569.