README.md - Update wiki pages link

As we moved the docs to a new repository, we need to update the README.md too.

Check our [wiki pages](https://gitlab.torproject.org/web/community/wikis/How-to-develop-on-the-website).

Should be: https://gitlab.torproject.org/tpo/web/wiki/-/wikis/How-to-develop-on-the-website

file: https://gitlab.torproject.org/tpo/web/community/-/blob/master/README.md