Update Diataxis landing pages

[a-velasco]

This PR updates Diataxis pages to comply with Canonical's Documentation standards for landing pages:

• Standardizes structure and formatting across all index pages (toctrees instead of links, categories)
• Addes or updates sentences with a concise summary of what to expect from each section or group of pages

Additional changes:

• Changed some links to safer Sphinx syntax ({ref} instead of []())
• Moved tutorial/index.md to tutorial.md, since it is a single page

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.