move pa11y.json in the .sphinx folder

[ru-fu]

This is a follow-up to #194 after I figured out how to test pa11y. ;)

We don't need this file on the root level of the repo.

Fixes #183

[ru-fu]

Does it matter that pa11y.js changes to pa11y.json?

It doesn't - we have a pa11y.js that is the actual tool and a pa11y.json file that contains the configuration for the tool.