Update docu

[ManuelAlonso1]

Motivation

Updated the Mirra backend setup documentation. Added detailed steps for installing Nix, Devenv, Protobuf, and dependencies for Elixir and JavaScript Protobuf, along with instructions to properly start the services. >

[Nico-Sanchez]

Key steps include:

Adding the character name in CharactersManager.cs.
Activating the character in the configurator.
Enabling the character's portrait in Unity.
Setting up default skins in the backend if needed.

Also please update your PR description, because the steps you mention doesn't apply to this PR

[mFragaBA]

LGTM!