Merge papi and papi-docs?

[peetzweg]

Had to search for these docs in the polkadot-api github org. Imho it would be handier if it lives inside the actual polkadot-api repository as well. Making it as easy as possible to touch and improve the docs along the way.

[carlosala]

We thought about it multiple times, let's leave it on hold and think about it in the future.