Adding GitHub action for link check of user guide

[deining]

With #1172, link checking in the user guide was introduced, thanks for that @chalin. A comment given in this PR states:

This isn't my ideal setup because I'd like to avoid the makefile entirely

This PR follows up on this comment and proposes an approach that allows to eleminate userguide/Makefile introduced with #1172.
This is done by adding GitHub action that makes use of the htmltest link checker to check the links of the user guide.
In its current form, this link check action will be performed on each push to the main branch of the docsy repo (this can be adapted, of course). To see the results of the last workflow run(s), select the htmltest workflow in the Actions section of the GitHub web UI. On the same page, you can also disable the workflow if needed. To do so, press the button … right beneath the search field that allows you to filter workflow runs.

This PR added two URLs to the IgnoredURLs setting in the config file of the link checker. With this adapation, the link checker currently reports back green status for the link check:

I hope you like this PR. If opens up several possibilities to streamline the workflow e.g. for pull requests.

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