Update API documentation when API v4 becomes stable

[lexvand]

Mainly the section "DATA EXCHANGE" needs to be rewritten when API v4 becomes the stable version. May be prepared by starting a duplicate section specifically for v4.

[lexvand]

I've made some basic changes to convert to v4, especially in API technical and API functional. We need to go through the docs more thoroughly and see what else should be changed.

Also, we should add a guide with the most common use cases that require changes in scripts to convert from using API v4 instead of v3. Apart from the changes mentioned in the announcement email, the main topics to be worked out in examples are:

• Read and write Timeseries data (endpoint changed from /data to /events + some other changes)
• Raster-aggregates to rasters sub-endpoints (/point, /line, /curve, et cetera)
• Raster exports to GeoTIFF (different query parameters)