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.

Start by comparing the command_line.html and config_file.html documentation linked in the issue, then trace how these pages are generated from mypy's option definitions. Define which shared options can be centralized while retaining configuration-only options, and consider the work done when their CLI and configuration descriptions are consistent in one discoverable documentation structure.