Add `# Indexer Guide` sections to `milestone_reached` and `funding_checkpoint` events

[Kingsman-99]

Description

invoice_state_changed in contracts/split/src/events.rs already has a detailed # Indexer Guide comment explaining how to use it. Similar events like milestone_reached and funding_checkpoint lack this guide. Adding it would help off-chain tooling authors understand how to filter and decode these events.

Acceptance Criteria

• Add a /// # Indexer Guide section to milestone_reached in contracts/split/src/events.rs
• Add a /// # Indexer Guide section to funding_checkpoint
• Each section explains the topic filter, the meaning of milestone_bps / threshold_bps, and the data fields
• cargo doc builds without warnings
• cargo test passes

Context

• Target file: contracts/split/src/events.rs