Update public API usage docs to match grant milestone requirements

[christabel888]

Context: Grant milestone deliverable: "Open-source repository + documentation" for the public API.

Where:

• This repo's README.md, docs/ (per the earlier README rewrite, docs/SECRETS_MANAGEMENT.md already exists here as a model for doc depth)

What to do:

• Document the new v1 analytics endpoints (network, soroban, wallets, rankings, insights) for external consumers: auth requirements, rate limits (Add rate-limit tier for public analytics API consumers #41), example requests.
• Also grep for stale "explorer" framing in backend code comments/docs and update wording to match the new positioning (no route/path changes).

Suggested approach:

1. Document each new endpoint from this batch with: method, path, auth requirement, rate limit tier (Add rate-limit tier for public analytics API consumers #41), and one worked curl example with real-looking (not placeholder) response data.
2. Fold in the small "explorer -> analytics framing" comment cleanup (merged from the old Rename internal route docs/comments from explorer framing to analytics framing in src/api and docs #30) as part of the same docs pass.

Watch out for:

• Docs that go stale the moment an endpoint's response shape changes are worse than no docs -- consider generating examples from the actual OpenAPI schema (Add OpenAPI schema entries in src/openapi.rs for the summary endpoint #6/Update OpenAPI tags/grouping to match new IA: Overview/Assets/Wallets/Soroban/Rankings #32) rather than hand-writing JSON that can drift.

Definition of done:

• Public docs cover every new endpoint added in this batch with a working example request
• No stale "explorer" framing left in backend docs/comments