Move entries from api-guide page into their own distinct pages in /api-reference

[tburny]

With a growing number of types and definitions, the API reference page becomes quite lengthy and it is easy getting lost while reading.

Changes:

• The api-guide.md contents were moved into separate pages for Event, Command, EventStore and CommandHandler.
• Each API reference item now is its own item in the sidebar (easy to find)
• Add overview page at /api-reference. Clicking the "Next" link links to /api-reference/event as first page from the navigation
• For each page, use a similar structure of: Description text ("Overview"), _Usage, Definition, See also. This "template" structure makes it easier to find the thing one is looking for at first glance. The sidebar nav on the right supports this (looks quite the "same" for Event, Command, Event Handler and Command Handler)
• Keep the api-docs.md page and link to the moved contents, as Cool URIs dont change.

Open items (tbd):

• [] Naming: Is it command-handler.md or commandhandler.md? I am favouring the second as reference to the type name in the code
• [] Decide on order of Usage and Definition. While I think it is more convenient to skip the code definitions and see "how can I use it" first, it feels a bit strange like in "First definition, then usage".