Documented `psycopg` extra is not declared in pyproject.toml

`docs/userguide.rst` documents a `psycopg` extra:

```
25: * ``psycopg``: for the Psycopg event broker
35:     pip install apscheduler[psycopg,sqlalchemy]
```

`pyproject.toml` does not declare one:

```toml
[project.optional-dependencies]
asyncpg = ["asyncpg >= 0.20"]
cbor = ["cbor2 >= 5.0"]
mongodb = ["pymongo >= 4.13.0"]
mqtt = ["paho-mqtt >= 2.0"]
redis = ["redis >= 5.0.1"]
sqlalchemy = ["sqlalchemy[asyncio] >= 2.0.24"]
```

The published `4.0.0a6` agrees: no `psycopg` requirement in its metadata. `psycopg[binary]` appears only under `[dependency-groups] test`.

The docs look right and the extra looks missing, rather than the reverse. `src/apscheduler/eventbrokers/psycopg.py:17`:

```python
from psycopg import AsyncConnection, InterfaceError
```

Every other broker has a matching extra, and `versionhistory.rst` records adding this one.

`pip` does not fail on an undeclared extra, so the reader gets `sqlalchemy`, no `psycopg`, and an `ImportError` later with nothing pointing back at the install line.

No PR because the floor is yours to pick. Happy to send one if you name it.

Found with [docproof](https://github.com/melbinjp/docproof), which flagged this as the document being wrong.

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