Docs: add clarity to customize editor page

[AndyCarnevale]

Docs on https://docs.coderedcorp.com/wagtail-seo/customizing/customize-editor.html currently look like this:

Problem: The panel objects are not shown being imported from the SeoMixin class, so the code doesn't work on first try.

I propose changing to:

class MyPage(SeoMixin, Page):

    # Each is a list of panels.
    promote_panels = (
        SeoMixin.seo_meta_panels +
        SeoMixin.seo_menu_panels +
        SeoMixin.seo_struct_panels
    )

I would make a pull request, but it seems the documentation in this repo is not up to date with what is on the website:
https://github.com/coderedcorp/wagtail-seo/blob/2c0de930e8e6f1aad3494823725ee3eca3dc481e/docs/customizing/customize-editor.rst