Documentation build should be strict (especially in CI)

[ocaisa]

There are a number of info statements right now that are really more like errors:

INFO    -  Doc file 'blog/posts/2025/03/gpu-support-eessi.md' contains a link '../../../../site_specific_config/gpu.md#installing-full-cuda-sdk-optional', but the doc
           'site_specific_config/gpu.md' does not contain an anchor '#installing-full-cuda-sdk-optional'.
INFO    -  Doc file 'test-suite/ReFrame-configuration-file.md' contains a link '#RFM_PREFIX', but there is no such anchor on this page.
INFO    -  Doc file 'tutorial/configuration_hpc.md' contains a link '#in-memory-cache', but there is no such anchor on this page.
INFO    -  Doc file 'tutorial/performance.md' contains a link 'eessi/high-level-design.md#compat_layer', but the doc 'tutorial/eessi/high-level-design.md' does not contain
           an anchor '#compat_layer'.
INFO    -  Doc file 'tutorial/troubleshooting.md' contains a link '#cache', but there is no such anchor on this page.
INFO    -  Doc file 'tutorial/access/alternatives.md' contains a link '../configuration_hpc.md#alien-cache', but the doc 'tutorial/configuration_hpc.md' does not contain
           an anchor '#alien-cache'.
INFO    -  Doc file 'tutorial/access/proxy.md' contains a link 'client.md##minimal_configuration', but the doc 'tutorial/access/client.md' does not contain an anchor
           '##minimal_configuration'.

[ocaisa]

Hmm, we are already strict with make test, how do we promote these to errors?

[ocaisa]

https://github.com/manuzhang/mkdocs-htmlproofer-plugin?tab=readme-ov-file#raise_error_after_finish