Fix flow-diagram

docs: Fix flow diagram

Make the flow-diagram correctly show that Epicardium will create menu.py if none was found on the external flash. This means, the menu will always be run from menu.py.

docs: Update overview diagram

The 3rd core is not used for BLE as of now so we should not document it as such. This commit also updates the colors of Epicardium and Pycardium to match the rest of the docs & GitLab.