Improve documentation for templated/special algorithms and connectors

[dbogdanov]

Add documentation for templated/"special" algorithms and connectors (such as VectorInput, DiskWriter, RingBufferInput, etc.).

This may go to the algorithm reference, or a special page linked from the reference, so that they are easier to find. Right now, their description is scattered across a few places (VectorInput, special connectors).

Add example code of their use to the descriptions (for example, VectorInput contains a doc page in the reference, and it will be useful to have an example of its declaration and use on the same page).

[yyf]

@dbogdanov thanks for the special connectors doc, it's helpful. For streaming mode, is running algorithm.process() alone enough or it still requires network.run()? if that's the case, what needs to be passed into network when it's initialized, i.e. Network network(VectorInput)?