Add CI using GitHub Actions and htmlproofer. #5

	name: CI
	on:
	push:
	branches:
	- '*'
	pull_request:
	branches:
	- '*'
	merge_group:
	types: [ checks_requested ]
	jobs:
	ci:
	runs-on: ubuntu-latest
	steps:
	- name: Checkout
	uses: actions/checkout@v4
	- name: Set up Ruby
	uses: ruby/setup-ruby@v1
	with:
	ruby-version: 2.6
	# runs 'bundle install' and caches installed gems automatically
	bundler-cache: true
	- name: jekyll build
	run: bundle exec jekyll build
	- name: htmlproofer
	run: bundle exec htmlproofer _site/ --disable-external=true

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