feat: Adding Terminology docs

[yhakbar]

Description

Adds documentation on commonly used terminology in Terragrunt. This documentation should be expanded as folks ask questions about commonly used terms, and updated when we use terminology inconsistently.

TODOs

Read the Gruntwork contribution guidelines.

• [x] Update the docs.
• [x] Run the relevant tests successfully, including pre-commit checks.
• [x] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• [x] Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added Terminology docs.