[Dev support] MVP-238 Moved contents of mcash.sphinx.ext to mcash.sphinx

Jira review task: [MVP-238](https://mcashdev.jira.com/browse/MVP-238)

Immediately after merging this PR, [#418](https://github.com/mcash/mcash/pull/418) in mcash and [#10](https://github.com/mcash/mcash.id/pull/10) in mcash.id should be merged.