Document legacy `/api/alertcity` usage

## Description

The legacy alert-city API is referenced in the repository but has no dedicated usage documentation.

Document its compatibility contract, including the difference between separate epidemiological year/week parameters and the newer `YYYYWW` format.

### Required work

* Document the canonical route and trailing-slash behavior.
* Document required parameters:

* `geocode`;
* `disease`;
* `format`;
* `ew_start` and `ey_start`;
* `ew_end` and `ey_end`.
* List supported diseases and formats.
* Add bounded JSON and CSV `curl` examples.
* Add an example response and common HTTP errors.
* State that the endpoint is legacy and returns legacy field names.
* Document that the archived `tweet` field is no longer returned.
* Link to `/api/v1/alert-city/` as the normalized alternative.

### Acceptance criteria

* A complete example can be copied and executed against production.
* Parameter types and week semantics are explicit.
* Legacy and v1 contracts are clearly distinguished.
* Documentation matches endpoint tests.