docs: add docs/governance.md covering input.governance event surface

[wolf31o2]

Goal

Document the governance event surface in a dedicated docs/governance.md. Adder emits governance data when transactions contain Conway-era governance content, but there is no user-facing documentation for the event type, its structure, or filtering.

Files

• event/governance.go (read for the GovernanceEvent, GovernanceContext, and related struct definitions)
• filter/cardano/cardano.go (read for governance filter handling)
• docs/governance.md (new)

Changes

Create docs/governance.md with:

• Overview: when adder emits a governance event and what triggers it.
• Event type name: input.governance (renamed from chainsync.governance in PR feat: rename event types from chainsync.block/transaction to input.block/tr… #617).
• Event structure: each field of GovernanceEvent, GovernanceContext, and the embedded data structures (proposal procedures, voting procedures, DRep certificates, vote delegation certificates, committee certificates) with one-sentence field descriptions.
• Supported governance actions: list of the seven proposal action types with a short description of each.
• Filtering examples: address (treasury withdrawal, reward account, vote delegation stake credential), pool ID, and DRep filtering. Show CLI flags and YAML config form for each.
• Example output: a JSON sample of a governance event for at least one common case (DRep registration or proposal procedure).

Keep examples concise and tested. Cross-link from README where appropriate.

Verification

• New file renders correctly on GitHub.
• All filter command examples run without error against a working adder setup.
• All field descriptions match the current event/governance.go structs.

Dependencies

None.