[Enhancement] Make offline advisory DB support prominent in the README

[sonukapoor]

Summary:
Updates the README to make offline advisory database support much more visible and easier to understand for developers, reviewers, and enterprise users.

Closes #77.

What changed:

• added a top-level offline advisory DB callout near the beginning of the README
• expanded the main usage examples with offline sync and offline scan commands
• added a dedicated offline workflow section under network behavior and privacy
• highlighted offline advisory DB support in the feature list and supported workflows
• added guidance about scheduled advisory DB refreshes via cron, CI, or similar automation
• clarified that advisory DB freshness depends on the last successful sync
• noted future TTL-style freshness warnings as a follow-up improvement

Notes:
This PR is documentation-only.

It is intended to make the new offline/local advisory DB workflow more discoverable and easier to adopt.

Verification:

• reviewed README content locally