GitHub actions: deployment to GitHub Pages, link check

[deining]

This PR follows up and closes #161. It adds automated workflows for

• site deployment on GitHub pages
• link checking

For this purpose, two workflow files for GitHub actions were added. Workflow actions can be either run manually (via GitHub web UI) or on each push and/or pull request. Two sections were added to README.md, describing the newly available features and their configuration.

Note:: In order to allow the workflow to cache npm dependencies, packages.json.lock was added to the repo.

[deining]

A production build should not be hosted by two servers

I agree, and it was never my intention to deploy the example site twice.

My motivation for this PR is different: in the user guide we recommend using this repo as template when building a new site. We cannot assume, that all users host their site at netlify, some may opt for site hosting GitHub Pages. The Github Pages deployment workflow was added in order to make it as easy as possible for latter users to get their site deployed. We easily can (and should!) disable this workflow once my PR was merged. To sum up, this workflow has sample/demonstration character only and was never meant to be used for the actual deployment of the example site.

WDYT?

[froboy]

What if we moved these files from .github/workflows to something like .github/example-workflows so that they're in the repo, but not enabled out of the box. Then we could have some documentation instructing users to mv them to the proper directory if necessary, but they wouldn't do anything if they were not necessary.