Cleanup duplications for KVScheduler

[ondrej-fabry]

Ligato Docs currently contain 3 articles named KVScheduler just in the navbar.

• Tutorials - https://docs.ligato.io/en/latest/tutorials/05_kv-scheduler/
• Plugins - https://docs.ligato.io/en/latest/plugins/kvs-plugin/
• Dev Guide - https://docs.ligato.io/en/latest/developer-guide/kvscheduler/

The last two seem to have started from same article, but now they are clearly very different due some modifications. We urgently need to clean this up to avoid confusing users.

This issue should also open some discussion about what kind of documenation should be kept at Plugins and Dev Guide.

At the moment the KVScheduler article from Dev Guide contains several different kinds of information:

• history of KVScheduler
• basic concepts & terminology
• API structure
• REST API reference

I'm sure that this mix of information in single article just overwhelms the reader. Users are looking for specific type of information in the docs at one point in time.

To quote from the article What nobody tells you about documentation that I've shared before:

In fact, it’s extremely hard to maintain good documentation that doesn’t implicitly or explicitly recognise the quadrants of this scheme. The demands of each kind are different from those of the others, so any attempt at documentation that fails to maintain this structure suffers, as it’s pulled in different directions at once.

I think we should define and write down (I started some draft in wiki already) guidelines for writing docs and apply them to current docs.

@chrismetz09 @milanlenco @VladoLavor