Minimal example for #89

[matlupi]

This MR adds a minimal example to reproduce #89.

[matlupi]

Example of failure

% mkdocs serve                     
INFO    -  Building documentation...
INFO    -  Cleaning site directory
INFO    -  Doc file 'index.md' contains an unrecognized relative link '../../../tests', it was left as is.
INFO    -  Doc file 'index.md' contains an absolute link '/assets/hello-world.drawio.svg', it was left as is. Did you mean 'assets/hello-world.drawio.svg'?
INFO    -  Doc file 'nested/page1.md' contains an absolute link '/assets/hello-world.drawio.svg', it was left as is. Did you mean '../assets/hello-world.drawio.svg'?
INFO    -  htmlproofer: Unknown url-scheme "mailto:" detected. "mailto:[email protected]" from "index.md" will not be checked.
INFO    -  htmlproofer: Unknown url-scheme "mailto:" detected. "mailto:[email protected]" from "index.md" will not be checked.
INFO    -  htmlproofer: Unknown url-scheme "mailto:" detected. "mailto:[email protected]" from "index.md" will not be checked.
INFO    -  htmlproofer: Unknown url-scheme "mailto:" detected. "mailto:[email protected]" from "index.md" will not be checked.
WARNING -  htmlproofer: Unable to locate source file for: page3.html#REGISTER.FIELD1
INFO    -  htmlproofer: Retrying URL page3.html#REGISTER.FIELD1 from nested/page1.md after 2 seconds...
WARNING -  htmlproofer: Unable to locate source file for: page3.html#REGISTER.FIELD1
INFO    -  htmlproofer: Retrying URL page3.html#REGISTER.FIELD1 from nested/page1.md after 4 seconds...
WARNING -  htmlproofer: Unable to locate source file for: page3.html#REGISTER.FIELD1
INFO    -  htmlproofer: Retrying URL page3.html#REGISTER.FIELD1 from nested/page1.md after 8 seconds...
WARNING -  htmlproofer: Unable to locate source file for: page3.html#REGISTER.FIELD1
ERROR   -  htmlproofer: invalid url - page3.html#REGISTER.FIELD1 [404] [nested/page1.md]
[...]