Website: inter-link the protocol and spec documentation sites

[jorisvandenbossche]

Currently, the home page (https://data-apis.org/) points to the "DataFrame API" (https://data-apis.org/dataframe-api/draft/), but this then doesn't mention the interchange protocol (it only points back to the homepage).

So for a moment I was confusing thinking the interchange protocol documentation wasn't online anymore. But we still link from it from the README of this repo (https://data-apis.org/dataframe-protocol/latest/index.html), but we could make this a bit more visible / discoverable.

[rgommers]

Good point, thanks Joris. I'll try to fix that up.