xcodegen fixes for .help

[chucker]

Following #156, properly re-implement building the Help target.

c025ac50f1c2927f7ec802c8ae368bd37e9bdc78 makes a few changes to that regard:

• the Makefile should (probably?) copy the contents of _site, not the folder itself
• the _English.lproj folder probably shouldn't exist alongside English.lproj (without underscore)
• the bundle identifier was wrong
• check for the existence of _site/nav.js to skip building this target if possible
• conversely, if building this target, always run the Makefile

However, even with these changes, I get the dreaded "The selected content is currently unavailable". Which may just be a caching problem.