Refactor Configuration API

This MR refactors and stabilizes the configuration system of the codegen module. It adopts much code from pystencils-sfg!2 (merged), and once it is through, pystencils-sfg can be updated to inherit its config system from pystencils.

Closes #116 (closed)

codegen.config

• Introduce Option descriptor for config options with default values, validation, and explicit unsetting
• Introduce ConfigBase class and Category descriptor for safely modelling config objects and correct implementation of their deep-copying
• Clean up the option categories and option validation

backend

• Remove all mention of codegen.config from the backend module, making it exclusive to configuring the codegen driver

Documentation

• Cleanly separate API reference from user manual pages (finally!)
• Add docs for updated config system

docs/source/api/symbolic/assignments.md

+# Assignments and AssignmentCollection
+
+```{eval-rst}
+
+.. py:class:: pystencils.Assignment
+
+    Monkeypatched variant of `sympy.codegen.ast.Assignment`.
+    Represents an assignment of an expression to a symbol.
+
+.. autosummary::
+    :toctree: generated
+    :nosignatures:
+    :template: autosummary/recursive_class.rst
+
+    pystencils.AssignmentCollection
+```