doc: add license information for gen. files

[adrientetar]

Closes #12069.

cc @brson

[brson]

I r+'d this with the following rationale:

• docs which originated in-tree are already covered by the prevailing license, MIT/ASL2
• docs which originated out-of-tree (cheatsheet, bugreport, proj/lang/usage faq) are predominantly written by Mozilla employees, or other contributors of long-standing who are aware of the project's licensing. Contributions by others are minor and unlikely to cause legal problems. This documentation is all likely to be heavily rewritten over the next few months anyway.

I do still intend to relicense the wiki eventually, obtaining statements from all wiki contributors, which should cement the status of the latter.