Docs: make each doc index documentation self-contained #1603

Some features are not documented for some of the doc indices. For example, sub indexing is only mentioned in elasticindex documentation, even though it's supported in many other backends. Same with query building, filtering, etc.

We should 1. come up with a list of features/topics that each document index documentation should have and 2. fill the missing parts in the existing docs

Also this link doesn't work