Update README.md

This commit introduces a significant refactoring of the devcontainer configurations to provide clearer, more distinct security levels. It also adds a new, experimental \paranoid\ mode for maximum security.\n\nKey changes include:\n\n-   **Rename `hardened` to `airgapped`**: The `hardened` container is now `airgapped`, focusing on network isolation. The CI workflow has been updated to test for this.\n-   **Repurpose `isolated` as `hardened`**: The `isolated` container has been repurposed to be the new `hardened` container, providing a secure environment with network connectivity.\n-   **Introduce `paranoid` container**: A new, experimental `paranoid` container has been added. This provides the highest level of security with a read-only filesystem and network isolation.\n-   **Update `auditor` and `minimal` containers**: The `workspaceMount` for these containers now binds to the local workspace, disabling the previous `tmpfs` isolation.\n-   **CI and Documentation Updates**: The GitHub workflow and README have been updated to reflect these changes, with new tests for network and filesystem isolation.

(readme): Rewrited project structure

This commit introduces the `eth-security-toolbox` devcontainer, a pre-configured environment for smart contract auditing based on the popular Trail of Bits toolbox.

Key changes include:

- Added the `eth-security-toolbox` devcontainer.
- Updated the GitHub Actions workflow to include the new container in the CI build matrix.
- Updated `README.md` to document the new container and clarify the use cases for all available devcontainer configurations.