Tutorial 3: Create a model and generate code using the API page issues

[agarny]

From https://libcellml.org/documentation/latest/tutorials/tutorial3/index:

• [ ] The http://www.cellml.org/cellml/2 link should be replaced with a link to http://www.cellml.org/cellml/2.0#.
• [ ] The API-related links are broken.
• [ ] The Understanding units link is broken.