Issue 8 — Documentation and samples for event/log observation API

### Scope
* New page under /docs covering:
* The Web3j-based subscription model and why it's used (vs. mirror-node polling) — including the tradeoff that this depends on the Hashio JSON-RPC relay being available/reliable for the target network.
* How to build EventParam lists for an event's ABI signature, and the indexed/non-indexed distinction.
* How to call SmartContractClient.subscribeEvent(...) directly, and how to cancel a subscription via the returned ContractSubscription (unsubscribe()/close()/try-with-resources).
* Supported ledgers (Mainnet/Previewnet/Testnet/Solo) and their endpoints.
* Known limitations (e.g. indexed dynamic-type parameters — string, bytes — only expose their keccak256 hash in EventLog.indexedValues, per #3).
* Update root README.md's "Managed services" list.
* Runnable end-to-end example in both hiero-enterprise-spring-sample and hiero-enterprise-microprofile-sample, using the injected SmartContractClient directly (smartContractClient.subscribeEvent(...)) — no new annotations, consistent with how the samples already demonstrate other SmartContractClient methods.

### Acceptance Criteria
[x] Docs page builds cleanly with the existing GitHub Pages setup.
[x] A new contributor can follow the docs to get a working listener running against Hedera testnet with no other context.
[x] Both sample apps demonstrate subscribeEvent(...) end-to-end via the plain SmartContractClient API.