docs: add aztec sequencer advanced setup guide

[itenev]

Summary

This PR adds comprehensive documentation for integrating and running an Aztec Sequencer node within the eth-docker stack. The new guide is placed under Advanced use cases to reflect its specialized nature.

Key Changes

- New Documentation Page: Added website/docs/Usage/Advanced/AztecSequencer.md.
- Docusaurus Integration: Includes proper frontmatter (sidebar_position: 7, sidebar_label: Aztec Sequencer) for automatic sidebar generation.
- Step-by-Step Key Migration: Provides explicit, operator-safe instructions for the hybrid keystore approach required by Aztec (delegating 42-char ETH addresses to Web3Signer while keeping the 66-char BLS private key inline).
- Operational Guidance: Covers all prerequisites (200k AZTEC stake, publisher gas), environment variables (AZTEC_P2P_IP), and Grafana Alloy metrics verification.

Related

Companion code PR adding the actual compose files and templates to eth-docker:
- https://github.com/ethstaker/eth-docker/pull/2638

Testing

- Verified Docusaurus frontmatter syntax.
- Confirmed the file path and category structure match existing advanced use case documentation (e.g., Vero.md, SSV.md).