Add third-party real-time servers section to streams docs

[palkan]

Follow-up to #141.

Move third-party Turbo-compatible servers to a separate section and add AnyCable. This way, we, first, make a clear ("third-party") separation between Turbo docs and external resources as well as provide alternatives (not only Mercure, but also AnyCable) to users.

As an alternative, we can make this section dead-simple and just list the severs and link to their respective guides:

## Third-Party Real-Time Servers

- [Mercure](https://mercure.rocks/docs/ecosystem/hotwire)

- [AnyCable](https://docs.anycable.io/guides/hotwire)

The current version is more informative though (transports, features, etc.).