docs: add cross-references to IC Dashboard APIs in relevant sections

[marc0olo]

Context

PR #209 adds docs/references/ic-dashboard-api.md covering all five public ICP Dashboard REST APIs:

• IC API (ic-api.internetcomputer.org/api/v3/) — metrics, nodes, subnets, governance, canisters
• Metrics API (metrics-api.internetcomputer.org/api/v1/) — time-series metrics
• ICRC API (icrc-api.internetcomputer.org/api/v1/) — ICRC token data
• Ledger API (ledger-api.internetcomputer.org/api/v1/) — ICP ledger data
• SNS API (sns-api.internetcomputer.org/api/v1/) — SNS/governance data

Task

Add contextual cross-references from the relevant doc sections to the APIs they'd use most:

• Tokens / ledger pages (PR docs: Batch 8 — tokens, ledgers, and cycles concepts #215 — docs/concepts/tokens-ledgers): link to ICRC API and Ledger API from ledgers.md or a dedicated digital assets guide
• Governance pages (PR docs: Batch 7 — governance deep dives #216 — docs/concepts/governance-deep-dives): link to SNS API from governance or SNS concept pages
• Protocol canisters reference (docs/references/protocol-canisters.md): link to IC API and Metrics API from the relevant sections (Bitcoin metrics, network metrics)
• Chain-key tokens / digital assets guides: link to ICRC API for token balance and transaction queries

The reference page is at docs/references/ic-dashboard-api.md and is already linked from the references index and the performance concept page.

[marc0olo]

All four tasks addressed in PR #208 (infra/learn-hub-migration-prep):

• ledgers.md: links added to ic-dashboard-api.md#icrc-api and ic-dashboard-api.md#ledger-api
• governance.md: links added to ic-dashboard-api.md#sns-api and ic-dashboard-api.md#ic-api
• protocol-canisters.md Next steps: ic-dashboard-api.md#ic-api added
• network-economics.md: ic-dashboard-api.md linked via exchange rate canister context

Will close on merge of #208.