[Docs] Clarify installation instructions and KV backend options in README

[yJader]

This pull request updates the README.md to clarify installation and launch instructions, and to update the default configuration documentation for backend selection. The most important changes are:

Documentation improvements:

• Clarified the recommended way to run tests by specifying the full path to pytest within the TeleFuser virtual environment, ensuring consistency with the installation instructions.
• Added a note explaining how the service launcher script resolves the Python interpreter, and how to override it if the repository layout differs.

Configuration documentation updates:

• Changed the documented default for kv_store_type from fluxon to local_file, and clarified that local_file is the preferred backend for local use, while Fluxon is for distributed deployments.