Add FAQ entry about tab completion in prompts

[kdeldycke]

This PR add a FAQ entry to:

• show users how to use readline to customize completion within prompts
• point them to Python documentation for more details about readline behavior on different platforms
• add a list of popular third-party interactive input libraries for users looking for fancy features

Closes #173 that is open since 2014. Its original request, completion when prompting for a path, works today: Click's prompt reads through input(), so readline handles the line.

Fancier interactive input at a prompt has been declined before:

• Use prompt_toolkit.prompt if available #1122
• fzf choice support #2047
• Add Navigable Choice Menu within click.prompt() #2795
• Allow a type to customize the prompt #1411 (still open about a customization hook)