Improve rendering of REST docs

[segfaultxavi]

I'm not used to REST API docs, so maybe it's just me, but I find the docs hard to navigate. Specially when there's Any of fields with lots of options. It might be worth trying to find a different template or renderer for the OpenAPI spec because other people has complained already.

[orisdorch]

Glad to know it's not just me! Heck - even an alternate rendering that doesn't collapse content would help.. I know it would be cluttered, but at least nothing would be hidden by default.

[segfaultxavi]

I feel you, I feel you... we'll get to this eventually 😁

[fboucquez]

Another openapi https://github.com/symbol/statistics-service/pull/75 to improve the doc style