Update Diataxis landing pages

[a-velasco]

Port of canonical/postgresql-operator#1260

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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.

[carlcsaposs-canonical]

nit: "higher" is not necessarily accurate; higher revision can be semantically older (and using "higher" language, imo, causes users to think of revisions as semantically sequential)