fix(identity): M0 and M1 reference page updates from feedback

[cloudjumpercat]

Description

Fixes #issue

Preview Links

• Glossary (updated credential entry)
• Updated the auth methods section (reused in basic auth, key auth, and key auth encrypted, made sure this won't show up in key auth encrypted)
• Added FAQs for how Kong Identity stores data
• Auth server reference (repurposed the old Kong Identity reference page so it's the reference for auth servers, clients, claims, and scopes now, please double check the use case table accuracy)

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

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[juliamrch]

Added some suggestions from @ajessup feedback (thank you!). For principals capitalization, I've planned to take it to the team in order to update the style guide and apply these changes if it fits it.