Enable linkchecker

[paulremo]

To avoid broken links in the handbook, a linkchecker should be enabled as part of the compilation check (related to #48).

The necessary changes are

• [x] Add linkchecker to the requirements.txt file
• [x] Add the following command to the compile_jupyter_book.yml file: linkchecker 02_NRPMHandbook\_build\html\home.html

This change will break the site and should only be implemented when enough time is availble to fix all broken links on the site.

[paulremo]

This works since 312c3d5e9e670d87a96ab0cc0bd448f8cfd68069.