Return `cleaned_data` for more consistent subclassing.
Updated documentation with an example of adding custom page validation for form fields.
Add unit test to ensure that the documented usage of extending `WagtailAdminFormPageForm` works as expected.
Originally from #10375
- Changed all British spellings to American spellings, in the Customizing admin templates section of the docs
- Transferred all British spellings in the usage guide and advanced topics section to American spellings
- Update a series of other pages to use the spelling cutomize in the file name
Implemented all feedbacks on the usage guide and advanced topics section
John-Scott Atlakson