need examples for creating & working with files

[b5]

we should define:

• a short explanation for what a file is, and how it differs from os.File
• documentation around how consuming a file's "reader" is a one-time thing
• basic usage patterns for working with a file as example funcs
• examples of working with "directory files"
• examples that have an eye toward using files in tests, which often involve creating & examining files on the spot

shoutout @dustmop