Add linkchecker script, result and badges

[ThomDietrich]

A first implementation for #565

For now only covers the check for broken links based on linkchecker. A script is provided to execute, it will execute linkchecker against the public page, then store an html report and update badges in README.md. Currently the results will need to be committed, that could be avoided whenever #520 is reconsidered.

I'll add the remark linting process in a later PR.

The included report was created with LinkChecker 8.6
I am waiting for the soon to be available pypi release from: https://github.com/linkcheck/linkchecker/releases/tag/v9.3.1

Signed-off-by: Thomas Dietrich Thomas.Dietrich@tu-ilmenau.de

[Confectrician]

Looks good so far.

I will do a test too and approve then.

[ThomDietrich]

@Confectrician you don't have to hurry testing. I'll add a better version of the LinkChecker results and append the remark tests soon. The script is basically working and if there is a bug somewhere it will pop up soon enough.

Note to myself: Maybe add a badge/text to inform about the last update date

[ThomDietrich]

Merging to have a first version out there. Follow-up PRs will come.

@kaikreuzer from now on, please execute the script alongside update-external-resources.sh. Thanks!

[kaikreuzer]

Will try to remember :-)