Add instructions on how to setup the Client Secret for authentication

[misha130]

Since the project is using secrets, the Client Secret for the authentication should be managed by the secrets as well.

An instruction on how to do this should be added to:
https://github.com/codidact/core/blob/develop/docs/linux-instructions.md
https://github.com/codidact/core/blob/develop/docs/windows-instructions.md

The client secret placeholder can also be removed from https://github.com/codidact/core/blob/develop/src/WebUI/appsettings.json

[asynts]

I wanted to rewrite the documentation anyways. It should be structured a bit differently in my opinion. I can add that while doing that.