Fix 404 links in documentation

[Siedlerchr]

Error:  http://www.math.uiuc.edu/K-theory/aux/K-theory.bib | Failed: Network error: error sending request for url (https://math.illinois.eduk-theory/aux/K-theory.bib)
[404] http://texdoc.net/pkg/btxdoc | Failed: Network error: Not Found
[404] http://texdoc.net/texmf-dist/doc/bibtex/tamethebeast/ttb_en.pdf | Failed: Network error: Not Found
Error:  http://www.math.uiuc.edu/K-theory/aux/K-theory.bib | Cached: Error (cached)
[404] http://texdoc.net/pkg/btxdoc | Cached: Error (cached)
Error:  file:///home/runner/work/user-documentation/user-documentation/.gitbook/assets/expand-firstnames-step-4.png | Failed: Cannot find file
Error:  file:///home/runner/work/user-documentation/user-documentation/.gitbook/assets/expand-firstnames-step-1.png | Failed: Cannot find file
Error:  file:///home/runner/work/user-documentation/user-documentation/.gitbook/assets/expand-firstnames-step-2.png | Failed: Cannot find file
Error:  file:///home/runner/work/user-documentation/user-documentation/.gitbook/assets/expand-firstnames-step-3.png | Failed: Cannot find file
[404] https://www.nlm.nih.gov/bsd/licensee/elements | Failed: Network error: Not Found
Error:  file:///home/runner/work/user-documentation/user-documentation/en/.gitbook/assets/AiPreferences.png | Failed: Cannot find file
Error:  https://console.mistral.ai/billing/subscribe/ | Failed: Too many redirects: error following redirect for url (https://auth.mistral.ai/self-service/login/browser?aal=&refresh=&return_to=&organization=&via=)
Error:  https://console.mistral.ai/api-keys/ | Failed: Too many redirects: error following redirect for url (https://auth.mistral.ai/self-service/login/browser?aal=&refresh=&return_to=&organization=&via=)
Error:  https://auth.mistral.ai/ui/login | Failed: Too many redirects: error following redirect for url (https://auth.mistral.ai/self-service/login/browser?aal=&refresh=&return_to=&organization=&via=)
[404] https://www.youtube.com/watch?v=2PpLZTol9 | Failed: Network error: Not Found
[404] https://dev.mysql.com/doc/connector-j/8.0/en/connector-j-reference-using-ssl.html | Failed: Network error: Not Found
[404] https://www.nlm.nih.gov/bsd/mms/medlineelements.html#pmid | Failed: Network error: Not Found
[404] https://info.diva-portal.org/about-diva/ | Failed: Network error: Not Found
Error:  https://bib-bvb.de/ | Failed: Network error: error sending request for url (https://bib-bvb.de/)
Error:  https://liinwww.ira.uka.de/bibliography/ | Failed: Network error: error sending request for url (https://liinwww.ira.uka.de/bibliography/)
[404] https://translate.jabref.org/ | Failed: Network error: Not Found
[404] https://translate.jabref.org/ | Cached: Error (cached)
Error:  http://www.tablesgenerator.com/markdown_tables | Failed: Network error: error sending request for url (https://www.tablesgenerator.com/markdown_tables)
[404] http://www.fileformat.info/info/unicode/block/mathematical | Failed: Network error: Not Found
[404] https://www.youtube.com/watch?v=2PpLZTol9 | Cached: Error (cached)
Error:  file:///home/runner/work/user-documentation/user-documentation/en/.gitbook/assets/specialfields-6columns-5.2%20(10).png | Failed: Cannot find file
[404] http://www.fileformat.info/info/unicode/block/mathematical | Cached: Error (cached)

[Siedlerchr]

most of them are fixed. Some of them are just lychee issues