Add an event schema reference for indexers

[Esc1200]

Adds docs/events.md, a full event schema reference for integrators building indexers.

What it covers

• All 7 contract events: SplitCreated, SplitPaid, SplitUpdated, SplitClosed, ControlTransferred, Deposited, Distributed.
• The topic/data layout of each event: the snake_case name symbol as topic 0, the split id (#[topic]) as topic 1, and the remaining fields serialized into the data map.
• A per-event table plus per-event detail (which entrypoint emits it, topics, data fields).
• A TypeScript decoding snippet using scValToNative matching how the dashboard reads events, and a note that indexers can filter by contract id and follow one split via the id topic.

Linked from the README Docs section. Docs-only change; no contract behaviour changes.

Fixes #138