Add documentation for Socket Services

[pdgendt]

Is your enhancement proposal related to a problem? Please describe.

Zephyr introduced Socket services in #66758, and are the go-to way of implementing various server-side network protocols.
However there is no dedicated documentation which makes it difficult to properly add new services.

Describe the solution you'd like

Dedicated documentation for socket services, on how to get started, handle special cases, a bare minimal example, ...

Describe alternatives you've considered

N/A

Additional context

N/A

[clamattia]

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