fix urls to reflect organisation github website

[apoorvalal]

Noticed a few pages on the documentation site still point to s3alfisc and are now broken
e.g.

For more details see the vignette on Difference-in-Differences Estimation
at the bottom of quickstart.html.

[s3alfisc]

Thanks, this was a lot of broken links! Should be fixed now. I updated some in the readme.md a few days before, but somehow completely forgot to consider all the other URLs sprinkled throughout the repo... 😅