[auditbox] update api docs

[Gerivian]

Note

Refactors and expands documentation for clarity and discoverability.

• Guides: Replace getting-started/using-backend.md with getting-started/send-auditevents.md and new getting-started/read-auditevents-using-api.md (auth, examples, pagination)
• TOC: Update SUMMARY.md to include the new guides
• API reference (api.md): Add explicit pagination parameters (_offset, _count) and remove verbose inline search/pagination examples to streamline the page
• Release notes: Add entries for 0.5.0 (custom user search parameter) and 0.4.0 (remove references to Keycloak)

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