Break up API reference into per-module pages (match docvet pattern)

[Alberto-Codes]

Problem

The API reference currently renders as one long page for the entire module index. Docvet breaks its API reference into separate per-module pages, which is easier to navigate and faster to load.

Solution

Update scripts/gen_ref_pages.py and/or the mkdocstrings configuration to generate separate pages per module:

• adk_secure_sessions (top-level exports)
• adk_secure_sessions.protocols
• adk_secure_sessions.exceptions
• adk_secure_sessions.serialization
• adk_secure_sessions.backends.fernet
• adk_secure_sessions.service (EncryptedSessionService)

Acceptance Criteria

• Each public module has its own API reference page
• Navigation sidebar shows per-module structure
• Cross-module links work correctly
• mkdocs build --strict still passes