Sorry again for another big docs PR. This time, I'm cleaning up the
config file docs.

Here are a list of notable changes made:

1.  I moved the example to come before we list the options, not after.

    I feel this makes the first section of the page more digestible: we
    have a big block of text, but at least now we can see some of that
    information in practice immediately after.

2.  I combined the two examples into one (and rewrote the explanations
    in list form, to help mitigate the increase in complexity).

3.  I swapped the global-only and per-module sections.

    After auditing the options, it seemed to me that the global-only options
    were generally less useful then the per-module options. I also think
    it makes sense to present the more flexible options first, rather
    then last.

4.  I organized the options into the same categories I introduced when
    organizing mypy's command line interface.

    Thankfully, most categories were entirely per-module or entirely
    global-only, with a few exceptions.

5.  I switched to using the same definition list format we use in the
    command line docs for consistency.

6.  I removed the `dump_type_stats`, `dump_inference_stats`,
    `debug_cache`, and the `strict_boolean` options.

    These are all options we have either suppressed or deprecated.

7.  I added in documentation for the `python_executable`,
    `no_site_packages`, and `disallow_untyped_decorators` options.

8.  I renamed some links/deleted some now-unnecessary links.

One change I made that was unrelated to the config file was adding docs
about the `mypy_path` and the per-module `ignore_missing_imports`
options to the "Running mypy" page.

Some interesting inconsistencies I noticed (but took no action on, in a
probably-futile attempt to keep this diff small):

1.  The `warn_redundant_casts` section is global-only, not per-module
    like the other warning options.

    I left this option in the global misc section for now.

2.  There does not appear to be any way of writing a config file that
    does the same thing as `--no-site-packages`.

    I think we should either add an option for this or allow expressions
    like `python_executable = None`. (Currently, we try parsing `None`
    as a string -- this makes mypy promptly crash with a
    FileNotFoundError).

Final note: I mostly kept the descriptions the same/minimal, apart from some
minor wording tweaks here and there -- I wanted to have the command line
docs act as the main source of truth.