docs(tap): Update GraphTally guide for Graph Horizon (#1059)

* docs(tap): remove outdated reference to Scalar

* docs(tap): update GraphTally guide for post-Horizon

   - Remove legacy V1 content (contracts,
   separate escrow subgraph)
   - Add Graph Horizon context and link to
   overview
   - Simplify to Horizon-first configuration with
    horizon.enabled = true
   - Add GraphOps gateway aggregator endpoint
   - Document operator_mnemonics for key rotation
   - Streamline architecture overview and getting
    started
   - Remove migration framing in favor of current
    setup guide

* docs(tap): add Horizon contract addresses

   - Add TAP Verifier, Subgraph Service, and
   Escrow contract addresses
   - Update config example to use Horizon V2
   addresses
   - Include both mainnet and testnet addresses

* docs(tap): fix Horizon config with V1/V2 TAP verifier addresses

   Add missing receipts_verifier_address_v2
   required when horizon.enabled = true.
   Without this, indexer-service-rs fails at
   startup. Also clarifies V1 vs V2
   verifier distinction in contracts table and
   adds note about escrow subgraph.

* docs(tap): add Horizon (TAP V2) migration guide

   Document the critical requirement that both indexer-service-rs
   and
   indexer-tap-agent must have Horizon enabled for V2 receipts to
   work.
   Includes configuration checklist, log verification examples, and
   troubleshooting for common misconfigurations.

* Apply suggestion from @benface

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Co-authored-by: Benoît Rouleau <benoit.rouleau@icloud.com>

Author: suchapalaver