Remove type hints Sphinx extension

[marfox]

The sphinx_autodoc_typehints extension only generates hyperlinks for objects in the Python standard library, leaving all the rest unclickable.

• [ ] remove sphinx-autodoc-typehints==1.6.0 from requirements.txt;
• [ ] remove the 'sphinx_autodoc_typehints' declaration from docs/conf.py;
• manually add types with explicit hyperlinks to docstrings in all modules:
• [ ] importer;
• [ ] ingester;
• [ ] linker;
• [ ] validator;
• [ ] wikidata;
• [ ] commons, requires #291.