Documentation update for issue #108

[quantstruct-canvas]

Attempt to resolve issue 108

The user wants the NGINX product glossary expanded with comprehensive definitions, usage notes, examples, and consistent terminology, including cross-links between related terms. To achieve this efficiently, we should introduce a centralized glossary page and update several existing docs to link to it and reference glossary terms. This ensures consistency across the docs and provides a single source of truth for terminology. Plan:

• Create a new glossary document (content/nginx/glossary.md) with initial terms and cross-link structure.
• Update the main index and key documentation to reference and link to the glossary.
• Add glossary references to a few representative docs (basic functionality, installation guide) and to the concept/template guidance, ensuring cross-links are visible.
• Update the concept/template guidance to encourage glossary usage and consistency.