docs: initialize git submodules

[leftkats]

Ποιο issue κλείνει το συγκεκριμένο PR;

Closes #1129

Περιγραφή του PR

This PR updates the setup instructions to include missing steps for submodule initialization.

Changes:

• Added a mandatory step to initialize and update submodules.
• Fixes the "File Not Found" error occurring during pip install -r requirements.txt.

The guide now correctly instructs users to run:
git submodule update --init --recursive
This ensures the cpython directory is populated with the necessary files before installing dependencies.

Validation:

• Verified that a clean clone successfully installs all requirements after running the submodule update command.