Add the examples/client_example to RTD

[kairoaraujo]

Description of issue or feature request:
An example can be a good start point for those reading the RTD Updater.
We could add the client_example in a way it can be explored by who wants to implement a client.

Current behavior:
The ngclient.Updater RTD does not have an example or point to the examples folder. The event folder is available only in the source code.

Expected behavior:
Easy landing or example demonstration to who wants to implement a client.

[lukpueh]

I think a sensible quick fix would be to just add a phrase or paragraph on RTD that links to the directories examples/client_example and examples/repo_example, akin to how it's done in the repo main readme.

This could be part of the RTD API reference and/or in a new top-level RTD "examples" page.