Add technical documentation for the package

- use sphinx to document the developed package
- create a nox session "docs" that builds the docs
- include a skeleton in the docs/ folder
  + how to install the package
  + how to use nox
  + license

setup.cfg

@@ -93,8 +93,13 @@ extend-ignore =
     WPS306,
     # Putting logic into __init__.py files may be justified.
     WPS412,
+    # Allow multiple assignment, e.g., x = y = 123
+    WPS429,
 
 per-file-ignores =
+    docs/conf.py:
+        # Allow shadowing built-ins and reading __*__ variables.
+        WPS125,WPS609,
     noxfile.py:
         # Type annotations are not strictly enforced.
         ANN0, ANN2,