Document lazy imports

[johnslavik]

Very much work in progress (took me 2 minutes, not everything is included and this likely has errors). I'll keep exploring different approaches here.

• Add a section on lazy imports in the import system
• Document __lazy_modules__
• Document __lazy_import__
• Document sys.lazy_modules -- waiting for Pablo's green light
• Add a crossref to lazy imports in importlib's "implementing lazy imports"
• Denote that types.LazyImportType cannot be accessed when _types is unavailable (types.LazyImportType can become types.ModuleType #43)