docs: apply Posit style guide fixes

[ian-flores]

Summary

• Add 'Posit' prefix to product names on first mention
• Expand acronyms (OIDC, SAML) on first use

Test plan

• Review documentation renders correctly
• Verify acronym expansions are accurate

[claude]

Claude finished @ian-flores's task —— View job

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Reviewing PR

• Read review guidelines
• Analyze PR diff and changes
• Create pending review
• Submit review with feedback

Review submitted. The documentation changes correctly apply Posit style guide conventions - adding the "Posit" prefix to product names and expanding acronyms on first use.

[claude]

Documentation style changes look good. The "Posit" prefix is correctly applied to Posit products while Keycloak (a third-party product) is left unchanged. Acronym expansions for OIDC and SAML in the authentication types table are appropriate since that's where they're first introduced to readers.

Minor observations (non-blocking):

• The expanded forms in the table make the "Type" column wider than "Value" and "Use Case", which slightly affects visual balance, but readability is fine.
• Line 61 in authentication-setup.md already had "OpenID Connect (OIDC)" after the table, so readers will see the expansion twice - this is acceptable for documentation clarity.