Move two remaining wiki pages that we refer to in our code.
When I was doing #40101, there were two wiki pages on trac.torproject.org for which I could not find replacements on gitlab:
https://trac.torproject.org/projects/tor/wiki/doc/TorFragileHardening (mentioned in configure.ac)
https://trac.torproject.org/projects/tor/wiki/doc/AChildsGardenOfPluggableTransports (mentioned in doc/HACKING/CircuitPaddingDevelopment.md)
These should find their way onto gitlab somewhere, and then we should update the links in the Tor repository.