check external links systematically.

[WolfgangFahl]

Some links in the glossary seem to be outdated/broken e.g.

http://iso25000.com/index.php/en/iso-25000-standards/iso-25010%7CISO in the Usability Quality Attribute term description

A systematic check in the pipeline might help to mitigate such problems

[gernotstarke]

see #91

[mattes3]

An alternative: Use a link caching service like https://permanent.link

Not only makes you aware of broken links but displays a cached version of the page when the link is broken.

[gernotstarke]

one might integrate the HtmlSanityChecker, or find another gradle-compliant service for checking...

(would help us to get rid of the old http-URLs spread all over the text)