docs: update top pages, add use case diagrams, refresh frontmatter and align with cards and nav

[miyoungc]

Description

• Improved the get started pages. Most of the content change are:
  • The installation page cleanup
  • Unify the shape of each tutorial by adding missing sections: prerequisites, verify guardrails, etc.
  • Generate a new Configuration Reference page based on the source code nemoguardrails/rails/llm/config.py and update the code samples.
• Added various AI-generated diagrams in mermaid and draw.io to resources.
• The rest are frontmatter refresh, and align the grid cards and the left nav titles with frontmatter.

Related Issue(s)

Checklist

• I've read the CONTRIBUTING guidelines.
• I've updated the documentation if applicable.
• I've added tests if applicable.
• @mentions of the person or team responsible for reviewing proposed changes.

[github-actions]

Documentation preview

https://nvidia-nemo.github.io/Guardrails/review/pr-1554

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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