Doc: refactor, prettify, clarify and improve performance of generated RPCs documentation

Context

This MR is mainly focused at reducing the cost of running the rpc_doc.exe generator which could take up to 5 minutes for the alpha protocol. This removes the generation/computation of JSON and binary schemas which were the main source of slowness.

Removing schemas required to adapt the UI which was also done in this MR. It is now, subjectively, better looking. The weird indented index arborescence is now flattened and intermediate non-endpoint services have been removed to leave only callable endpoints to ease comprehension. The CSS has also been slightly adapted and the javascript is no longer needed.

I also added a mention in the header that the list is not exhaustive. Indeed, some dynamic endpoints are not referenced and no mention of this was made. The documentation now advertise this fact and mentions the OpenAPI specification which is more complete and also contains schemas.

Manually testing the MR

Build the doc and compare with, e.g., http://tezos.gitlab.io/shell/rpc.html

The result can also be checked in the artefacts below. However, the build process is interesting in itself for this MR, to see the performance improvements.