docs: add glossary of terms to general reference

[Giggitycountless]

Description

Add a glossary section to the general reference docs defining key terms as they are used throughout the Click documentation. This helps maintain internal consistency and reduces ambiguity for new users.

Terms defined

• Application / Command line application — a complete CLI program built with Click
• Argument — a positional parameter
• Command — a single callable unit
• Command line utility — an application for a specific task
• Flag — a boolean option ()
• Group — a command containing subcommands
• Option — a named parameter ()
• Parameter — generic term for options and arguments
• Terminal — the text-based interface

Each term uses MyST cross-references (, , , ) to link to detailed documentation.

Fixes #3077