Decide on URLs

[mattmassicotte]

Slightly superficial, but the article file names show up in URLs. I guess these could be remapped as desired in a post-build step, but it also might be nice to just give this some thought before pressing the button.

Currenty:

• "Data Race Safety" -> /dataracesafety
• "Enabling Complete Concurrency Checking" -> /completechecking
• "Incremental Adoption" -> /incrementaladoption
• "Common Compiler Errors" -> /commonproblems

Also, the root is /swift-6-concurrency-migration-guide, which is derived from the CFBundleDisplayName entry in the Info.plist file.

[hborla]

cc @shahmishal was thinking about ways to configure or change the URLs