[epic] document MirrorNodeAccountBalanceQuery as the sdk-native migration path for AccountBalanceQuery

### background

`MirrorNodeAccountBalanceQuery` is an sdk-native replacement for the deprecated `AccountBalanceQuery`. it reads an account's hbar balance from the mirror node rest api (`GET /api/v1/balances`) while keeping the sdk query interface (`.setAccountId().execute(client)`), with automatic retries and no query payment. verified against `hiero-ledger/hiero-sdk-js` source at tag `v2.87.0` and the published `@hiero-ledger/sdk` npm package.

**availability** (verified against the sdk repos, not a roadmap claim):

| sdk | version | status |
|---|---|---|
| javascript / typescript ([`hiero-sdk-js`](https://github.com/hiero-ledger/hiero-sdk-js)) | v2.87.0+ | available now |
| java ([`hiero-sdk-java`](https://github.com/hiero-ledger/hiero-sdk-java)) | — | in progress |
| go ([`hiero-sdk-go`](https://github.com/hiero-ledger/hiero-sdk-go)) | — | in progress |
| rust ([`hiero-sdk-rust`](https://github.com/hiero-ledger/hiero-sdk-rust)) | — | no implementation, no announced timeline |

### documentation requirements

- **cover every language tab the pages carry.** `get-balance`, `tokens/get-balance`, and `queries` document javascript, java, go, and rust. rust has no replacement and no timeline (verified against `hiero-sdk-rust` source), so rust readers are routed to the mirror node rest api explicitly and never told "coming soon."
- **HBAR only.** `MirrorNodeAccountBalance` returns a single `hbars` field. it is not a token-balance query; `tokens/get-balance.mdx` must keep readers on the rest token-balances endpoint (#722).
- **terminology.** "public key alias" is a retired term enforced by the terminology check; use "evm address, or account alias."

### out of scope

- an in-repo "migrating from AccountBalanceQuery" guide (currently external).
- `reference/protobuf/crypto/cryptogetaccountbalance.mdx` (protocol-level, unaffected).