Add wiki/ to the repo and remove its content during cleanup

As the files under wiki/ may be generated by Apache, when you run ./site.sh mklive they cannot be overwritten. Ensure wiki/*.html files are removed during cleanup so this doesn't happen.

Also ensure that the wiki/ directory is always available by adding it to the repo with a placeholder README, so cleanup doesn't throw an error if it's missing. This is preferable anyway: currently we're relying on the directory being manually setup at install or run time, which could be missed.

Adding wiki/ to the repo also means we can remove a test for its existence in gfm.php (which is inefficient).