Add js test and docs back to CI

[ianhi]

For now this PR skips tests and docs, just so we can have a basic workflow working for other PRs. But we should eventually add them back, and also check the generated workflow from the cookiecutter is also correct.

Originally posted by @jtpio in #91 (comment)

[jtpio]

Thanks @ianhi.

For the tests, which might be fixed by #86:

widget-ts-cookiecutter/.github/workflows/build.yml

Lines 61 to 63 in 78716a6

	# TODO: re-enable tests
	# pytest
	# npm run test:ci

For the docs:

widget-ts-cookiecutter/.github/workflows/build.yml

Lines 65 to 70 in 78716a6

	# - name: Check docs can be build + links
	# run: |
	# pushd jupyter-widget-testwidgets/docs
	# make html
	# make linkcheck
	# popd

And for the generated cookiecutter workflow:

widget-ts-cookiecutter/{{cookiecutter.github_project_name}}/.github/workflows/build.yml

Lines 49 to 61 in 78716a6

	- name: Test the extension
	run: |
	python -m pip install --upgrade -v -e ".[test, examples, docs]"
	npm run lint:check
	pytest
	npm run test:ci
	- name: Check docs can be build + links
	run: |
	pushd docs
	make html
	make linkcheck
	popd
	python -m pytest_check_links --links-ext=.md -o testpaths=. -o addopts=