Add linkchecker for docs and non-docs files

[stevepiercy]

Currently we have make linkcheckbroken to display only broken and redirecting links in the documentation by virtue of Sphinx's linkcheck builder. However, this doesn't check links in other files that are not in the documentation, including the *.md and *.rst files at the root of the repo.

I've been using both lychee locally and lychee-action in GitHub workflows with Volto and other repos. It's insanely fast and has caught lots of issues. I'd suggest adopting it for icalendar and its related projects.

Sample CI configuration:

https://github.com/plone/volto/blob/main/.github/workflows/readme-link-check.yml

Sample local usage, copy-pasted options from above file and escaping line breaks:

lychee    --timeout 5 \
          --max-retries 2 \
          --exclude https://www.npmjs.com/ \
          --exclude http://localhost:8080 \
          --exclude http://localhost:3000 \
          --exclude https://github.com/kitconcept/volto-blocks-grid.git \
          --exclude https://my-server-DNS-name.tld/api \
          --exclude 2021.ploneconf.org \
          --exclude https://www.lanku.eus/ \
          --exclude https://medium.com/ \
          '**/README.md' \
          'PACKAGES.md'

To run lychee locally, you'd need to install it on your system, not just a virtual environment, so I never made a make command to run and install it.