Consider merging command line flags and configuration file options descriptions

Description for command line flags and configuration file options are today split across 2 different documentations (here for CLI flags, and here for options).

Despite the options mostly behaving the same whether they are defined through the CLI or a configuration file, most of the descriptions differ (for instance, here for --python-version vs. here for python_version). Additionally to the differences, it also requires to maintain documentation across 2 different places, and may make it harder for people to refer to specific options of mypy.

I'm not sure how feasible this would be, as some options may only be defined in configuration files (especially for module-tied ones), but maybe the "common" options could be merged into a single documentation?

While I think it is auto-generated from the code, isort documentation is a good example of a centralized documentation for options. For each option, both the CLI flags and configuration file option are specified, and examples for each possible configuration type is provided.

Beginne mit dem Vergleich der in der Issue verlinkten Dokumentation zu command_line.html und config_file.html und verfolge anschließend, wie diese Seiten aus den Optionsdefinitionen von mypy generiert werden. Lege fest, welche gemeinsamen Optionen zentralisiert werden können, während nur für die Konfiguration geltende Optionen erhalten bleiben, und berücksichtige den Aufwand, der anfällt, wenn ihre CLI- und Konfigurationsbeschreibungen in einer einzigen auffindbaren Dokumentationsstruktur konsistent sind.