documentation: switch all hyperlinks in the documentation to https://

[tamiko]

It is 2023... This pull request switches all external urls from http:// over to https://. Otherwise, modern browsers will complain loudly about our website...

Note that some (for example most in the doc/news/ subdirectory) are probably dead links. But let's convert them nevertheless. It's better if a browser fails at trying to establish a secure connection than an insecure one and fail afterwards with an http 404.

[bangerth]

Is there a way to determine automatically which links are dead?

[tamiko]

@bangerth Something like this?

cd doc && grep -ohRP "https://[-a-zA-Z0-9@:%_\+.~#?&\/=]*" | while read -r url; do curl --silent --head --fail --output /dev/null "$url" || echo "Potentially broken: $url"; done
Potentially broken: https://github.com/gentoo/sci/blob/master/sci-libs/dealii/dealii-9.1.0.ebuild
Potentially broken: https://www.dealii.org/publications.html#details
Potentially broken: https://www.dealii.org/current/doxygen/
Potentially broken: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/
Potentially broken: https://epubs.siam.org/doi/10.1137/16M110455X
Potentially broken: https://doi.org/10.1016/S0045-7825
Potentially broken: https://doi.org/10.1016/s0045-7825
Potentially broken: https://doi.org/10.1016/S0168-9274
Potentially broken: https://doi.org/10.1016/0045-7825
Potentially broken: https://doi.org/10.1016/s0301-9322
Potentially broken: https://doi.org/10.1016/0020-7683
Potentially broken: https://epubs.siam.org/doi/abs/10.1137/1.9781611971200
Potentially broken: https://www.sciencedirect.com/science/article/pii/S0021999121006148
Potentially broken: https://academic.oup.com/imajna/article/43/2/627/6517084
Potentially broken: https://doi.org/10.1016/S0168-9274
Potentially broken: https://doi.org/10.1016/S0045-7825
Potentially broken: https://doi.org/10.1016/S0045-7825
Potentially broken: https://doi.org/10.1002/
Potentially broken: https://doi.org/10.1016/0020-7683
Potentially broken: https://doi.org/10.1016/0045-7930
Potentially broken: https://doi.org/10.1016/0377-0427
Potentially broken: https://ginkgo-project.github.io/ginkgo/doc/develop/index.html
Potentially broken: https://muparser.beltoforion.de/
Potentially broken: https://computation.llnl.gov/projects/sundials
Potentially broken: https://www.dealii.org/9.5.0/doxygen/deal.tag
Potentially broken: https://www.dealii.org/8.5.0/doxygen/deal.tag
Potentially broken: https://www.dealii.org/X.Y.Z/doxygen/deal.II
Potentially broken: https://validator.w3.org/check?uri=referer
Potentially broken: https://jigsaw.w3.org/css-validator/check/referer

Might contain some false positives.